Poll: what makes a good diplomacy system?

[Winston Hughes]

My view is

1. The AI should be designed to Win. period.

2. The GAME should be designed so that Players (whether AI or human) that are trying to win will tend to behave like 'real countries' (not attacking their allies, attacking those that attack their friends, attacking those that they have social disagreements with, etc.)

Basically, AI needs to be deleted from those options

Diplomacy is better if Players where you have very good relations are unlikely to attack you
Diplomacy is better if Players play to win, including attacking their close allies

I would vote for both.

#2 needs to be applied to the AI (by changing the AI so that it is totally ruthless)
#1 needs to be applied to Humans (by changing the game so that attacking your ally has some problems)

I'm beginning to think there should be an "I agree with Krikkitone" smiley.

100% spot on, as usual.

 

[Che Guava]

I'd be happy to see a general modifier that applies to all relations you or the AI has with all other civs that relates to 'trustworthiness', i.e. if you backstab a close ally, ALL AI civs are less likely to have good relations with you, which would be a good deterent to underhandedness.

Apologies if that actually does exist, I'm not all too familiar with how the modifiers work.

 

[Ahriman]

I do like the idea of in game modifiers to encourage human players to make real world logical diplomatic decisions

i.e. if you backstab a close ally, ALL AI civs are less likely to have good relations with you

I agree that its a nice idea, but the devil is in the details.

What does it mean for someone to be a close ally?

In Civ4, each faction has a diplomatic opinion of each other faction, except for the human player, who has no such diplomatic values.

Your diplomatic reliations with faction X are what THEY think of YOU, not what you think of them.
Its entirely possible in-game for faction A to really like faction B, while faction B hates faction A. If A declares war on B, then B gets mad at A, but A doesn't get mad at B.

Thus, it is impossible to define things like "close ally" without creating a preference for the human player's faction over countries (unless we were willing to say that our ally is anyone who likes us - no matter how we feel about them).

And now we get into difficulties.

We need to have preferences for the human player's population, which are totally indepdendent of the preferences of the actual human playing the game (ie you, who paid money to enjoy the game).

What determines those preferences?
We can't have them be controlled directly by the player, or they're meaningless (I just choose to hate the people I want to attack and like the people I don't).
Clearly they'll be influenced by actions taken by the AI (if Civ X attacks me, then my people will get angry at them). Which is problematic, since I have no control over these.
We need to have them be influenced by the player's actions somehow.

How will that work?

The obvious candidate we have to influence them are our diplomatic actions.

But that creates a lot of strange incentives, and gets us into the weird and totally unrealistic situation where the human player uses foreign policy in order to change their own faction's preferences. I initiate trade sanctions against Civ Y - which makes my people like Y less? I attack Civ Z in order to make my people like them less, in order to make it easier to declare war on them in future?

We get very complicated very quickly, and the system becomes highly incoherent.


* * *
The other alternative would be to make diplomacy values bilateral rather than unilateral. So there is no longer any difference between what A thinks of B and what B thinks of A. This has a lot of problems too; how do we model things like the impact of A running B's favorite social policy/civic (positive modifier), while B is running A's least favorite social policy/civic (negative modifier)? We lose a lot of the fine-detail ability here, and we lose a lot of player control.

Still, maybe this is the best alternative.

 

[KrikkitTwo]

The obvious candidate we have to influence them are our diplomatic actions.

But that creates a lot of strange incentives, and gets us into the weird and totally unrealistic situation where the human player uses foreign policy in order to change their own faction's preferences. I initiate trade sanctions against Civ Y - which makes my people like Y less? I attack Civ Z in order to make my people like them less, in order to make it easier to declare war on them in future?

We get very complicated very quickly, and the system becomes highly incoherent.

That actually makes Perfect sense, it means every diplomatic action tends to force you into a path.

Also those people's reactions can be controlled by non-diplomatic means (building more temples, spending more on culture, switching government to one that gives more happiness) The player does that as a normal part of the game, it isn't specific, but it helps the players in ALL their control of their population.

The other alternative would be to make diplomacy values bilateral rather than unilateral. So there is no longer any difference between what A thinks of B and what B thinks of A. This has a lot of problems too; how do we model things like the impact of A running B's favorite social policy/civic (positive modifier), while B is running A's least favorite social policy/civic (negative modifier)? We lose a lot of the fine-detail ability here, and we lose a lot of player control.

Still, maybe this is the best alternative.

Assuming there are most/least favorite "civics" there is a VERY easy way to do it

The player gets a bonus for adopting that civic (or the AI is more likely to adopt that civic if we want those only to apply to the AI)

Then the Penalty/Bonus is for having a Different/Same 'civic'

So Instead of
England likes France because France is a Monarchy

you have
England is a Monarchy because England likes Monarchies
England likes the French Monarchy because France has the same government it does (and France will like England for the same reason.... even if France doesn't like Monarchies)

If England changes to a Democracy, then it will like other Democracies

So If England likes Monarchies and France Likes Democracies then

England will (probably) BE a Monarchy, France will (probably) BE a Democracy... so they won't like each other (unless gameplay considerations overweigh their preferences)

 

[Ahriman]

it means every diplomatic action tends to force you into a path.

Which is just crazy from any kind of real historic perspective. Countries don't undertake foreign policy in order to change the opinions of their domestic populations. The causal direction is wrong here.

Also those people's reactions can be controlled by non-diplomatic means (building more temples, spending more on culture, switching government to one that gives more happiness) The player does that as a normal part of the game, it isn't specific, but it helps the players in ALL their control of their population.

I don't understand your point here. All of these have simple impacts in Civ4. How could you use non-diplomatic means to change your population's perspective of another faction?
I build another temple; does that make my people like the Romans more or less?

England is a Monarchy because England likes Monarchies

I see how this would work, but I strongly dislike hardwiring these kind of preferences into the human-controlled player. The human player should be free to choose their preferences over "civics".
Maybe it could work even without this though.

* * *
So: remove unilateral diplomatic relations, have a single modifier between both sides.

If I declare war on you, our relationship gets worse. If I give you a gift, our relations get better. If we trade with each other, our relations get better. If I trade with A, who has bad relations with B, then my relations with B get worse.

Then have a mechanic where if I declare war on someone with whom I have good relations, I get a happiness penalty depending on how good our relationship is, and I get a relationship penalty with all civs if our relations were "friendly" or better.
[Which would be simpler; the penalty is based on my relations before suffering the hit from declaring war, or on my current relations at any given point in time? Eg if during the war I change "civics" to match yours, does that reduce my unhappiness?]

Still a few weird implications, like I might deliberately change "civics" away from you in order to make it easier to declare war on you.
But if social policies are hard to change (no equivalent to spiritual trait) then maybe this doesn't matter.

I guess I could see this working, and this would bridge a lot of the differences of opinion we see in these threads.

The remaining challenge would be for the AI to figure out how to value the costs of declaring war on a "friend".

 

[Che Guava]

I agree that its a nice idea, but the devil is in the details.

What does it mean for someone to be a close ally?


In Civ4, each faction has a diplomatic opinion of each other faction, except for the human player, who has no such diplomatic values.

Your diplomatic reliations with faction X are what THEY think of YOU, not what you think of them.
Its entirely possible in-game for faction A to really like faction B, while faction B hates faction A. If A declares war on B, then B gets mad at A, but A doesn't get mad at B.

I see what you're getting at, but it might be easier to put it in 'real' terms rather than attitude. Say, if you go to war immediately with someone with whom you have open borders and are trading resources, then you get a penalty.

Aside from that, why not just have a 'mock modifier' for human players. It might not dictate with whom you are actually friendly, but would at least say with whom you should be friendly, based on thier actions towards you. Use that to generate 'misbehaviour' points.

That actually makes Perfect sense, it means every diplomatic action tends to force you into a path.

Also those people's reactions can be controlled by non-diplomatic means (building more temples, spending more on culture, switching government to one that gives more happiness) The player does that as a normal part of the game, it isn't specific, but it helps the players in ALL their control of their population.



Assuming there are most/least favorite "civics" there is a VERY easy way to do it

The player gets a bonus for adopting that civic (or the AI is more likely to adopt that civic if we want those only to apply to the AI)

Then the Penalty/Bonus is for having a Different/Same 'civic'

So Instead of
England likes France because France is a Monarchy

you have
England is a Monarchy because England likes Monarchies
England likes the French Monarchy because France has the same government it does (and France will like England for the same reason.... even if France doesn't like Monarchies)

If England changes to a Democracy, then it will like other Democracies

So If England likes Monarchies and France Likes Democracies then

England will (probably) BE a Monarchy, France will (probably) BE a Democracy... so they won't like each other (unless gameplay considerations overweigh their preferences)

I think I understand you here, but just to be sure: modifiers between civs improve with similarity of civics, but (as usual) domestic reaction to said civics will depend on buildings, etc?

Sounds just great to me. Add a few other 'traditional' modifiers like resource-trading/open-borders/years-of-peace, and I think you have a pretty dynamic yet realistic diplomacy scheme.

 

[Ahriman]

Say, if you go to war immediately with someone with whom you have open borders and are trading resources, then you get a penalty.

Doesn't really work. I just cancel open borders and resource trades (without penalty) before I go to war. Unless you want to penalize the player for ending a trade for a resource they no longer need?

It might not dictate with whom you are actually friendly, but would at least say with whom you should be friendly, based on thier actions towards you.

I don't really like the idea of the game telling me who I should be friends with. I want to decide that as the player. What if I decide I want to hate the French, because they wear funny hats? Or to raze New York to take revenge for McDonalds?

I think I understand you here, but just to be sure: modifiers between civs improve with similarity of civics, but (as usual) domestic reaction to said civics will depend on buildings, etc?

I think what he's saying is:
Rather than having a diplomatic modifier only when I adopt your favorite civic, instead have diplomatic modifiers that depend on how far apart our civics are.
This might be even easier if social policies are more graded than civics, so I move along a democracy path in steps rather than just instantly transitioning between democracy and police state. Then we can base the diplomatic effects on the number of steps apart we are in social policies. Eg: if we are 0-5 steps apart across all civics, give +3 bonus, 6-10 steps apart give +1 bonus, 11-15 steps apart, no impact, 16-20 steps apart give -1 penalty, 21+ steps apart give -3 penalty. Or something like that.

Buildings aren't related. There is no "domestic reaction" to the civics, other than the direct effect of the civics themselves.

 

[Che Guava]

Doesn't really work. I just cancel open borders and resource trades (without penalty) before I go to war. Unless you want to penalize the player for ending a trade for a resource they no longer need?

Well, assuming that you couldn't cancel deals AND go to war in the same turn without a penalty, you could at least take the sting out of a surprise attack if the AI wanted to avoid looking like a villain on the global stage.

Or you could make it more nuanced by saying that you couldn't declare (unpenalized) war on ANYONE without a supposed justification, which would take the form of either a random event (one of those 'bride left at the altar" mishaps ), the uncovering of a espionage mission from your opponent, or trading with another civ with which you are currently at war.

I don't really like the idea of the game telling me who I should be friends with. I want to decide that as the player. What if I decide I want to hate the French, because they wear funny hats? Or to raze New York to take revenge for McDonalds?

Again, it doesn't have any actual consequences for who you trade with, sign alliances with, or invade. If you do decide to invade france over berets, baguettes, or smelly cheese, go for it! But if France just traded techs with you and emptied out its granaries to save one of your cities, you're being kind of a dick. And the rest of the world will treat you like one, at least for the next 20 turns or so...

I think what he's saying is:
Rather than having a diplomatic modifier only when I adopt your favorite civic, instead have diplomatic modifiers that depend on how far apart our civics are.
This might be even easier if social policies are more graded than civics, so I move along a democracy path in steps rather than just instantly transitioning between democracy and police state. Then we can base the diplomatic effects on the number of steps apart we are in social policies. Eg: if we are 0-5 steps apart across all civics, give +3 bonus, 6-10 steps apart give +1 bonus, 11-15 steps apart, no impact, 16-20 steps apart give -1 penalty, 21+ steps apart give -3 penalty. Or something like that.


Buildings aren't related. There is no "domestic reaction" to the civics, other than the direct effect of the civics themselves.

Which makes perfect sense to me: civs that have similar social policies are more likely to get along and domestic policy remains domestic.

 

[Chalks]

Imagine public opinion is a slow adjusting representation of your own attitudes towards other civilizations. If you are hostile towards a civ, your people will over time develop a hatred for them. If you are friendly with them, over time your people will begin to love them. The important thing is that their attitudes change slowly, and if you make decisions that go massively against popular opinion you will see riots etc.

The real world rationalisation for this is that if two nations are friendly, you will get lots of immigration between the nations. People will have friends, family and businesses over the boarder. If you suddenly declare war, you're going to have a population who are very angry and will try to stop you.

Equally, if you are very hostile towards a nation your people will resent them. If you try to become all friendly, the residual hostility of your people will cause things like members of the other nation being attacked in the streets and assassination attempts against high profile figures who represent the other nation. Trouble for you.

If you change your stance more gradually, the media and public opinion in your nation will be able to adjust accordingly.

Balanced in such a way that it only impedes sudden changes of heart and still allows for rapid diplomatic changes for civs that your people are relatively ambivalent about.

I could see something like that working.

 

[KrikkitTwo]

Then have a mechanic where if I declare war on someone with whom I have good relations, I get a happiness penalty depending on how good our relationship is, and I get a relationship penalty with all civs if our relations were "friendly" or better.
[Which would be simpler; the penalty is based on my relations before suffering the hit from declaring war, or on my current relations at any given point in time? Eg if during the war I change "civics" to match yours, does that reduce my unhappiness?]

".

It would probably be easiest to have the Happiness Penalties/Benefits happen only at the Change of a diplomatic state and then fall off.

Ie

Currently American-German Relations are at +3 (+2 we like you, +1 we have been at peace*)
America declares war on Germany
This causes
1. 6 unhappiness in America (+3 x 2) **
2. American-German Relations to Drop to -2 (+2 we like you, -2 war was declared**, -2 we are at war)
3. America and Germany suffer war weariness


So now if America and Germany Declare peace
1. 4 unhappiness in Both America and Germany (-2 x 2) **
2. American German Relations go to 0 (+2 we like you, -2 war was declared**, +0 we have been at peace*)
3. America and Germany are free from war weariness


*slowly builds up
**slowly fades away


Because
diplomatic states-> "Relations value"
and
diplomatic states + "Relations value" ->happiness

to prevent complex interrelationship then the happiness should only come from the change and the relations value Before the change. But the relations value should be based on the change AND the current state.

 

[Ahriman]

Well, assuming that you couldn't cancel deals AND go to war in the same turn without a penalty, you could at least take the sting out of a surprise attack if the AI wanted to avoid looking like a villain on the global stage.

But should the player or AI treat every cancellation of a trade deal as a semi-declaration war the next turn?
If not, then you haven't really changed much.

Or you could make it more nuanced by saying that you couldn't declare (unpenalized) war on ANYONE without a supposed justification, which would take the form of either a random event (one of those 'bride left at the altar" mishaps ), the uncovering of a espionage mission from your opponent, or trading with another civ with which you are currently at war.

This is Civilization, not Kumbaya-Hippieville

And I don't think we want to get into developnig a specific cassus belli system.

But if France just traded techs with you and emptied out its granaries to save one of your cities

Nothnig like these exists in Civ5. I take your point, but I still think that we don't want the player to feel like they're having to fight against the diplomatic preferences of their people.

civs that have similar social policies are more likely to get along and domestic policy remains domestic

Right, that's the idea.

It would probably be easiest to have the Happiness Penalties/Benefits happen only at the Change of a diplomatic state and then fall off.

This seems overall reasonable. I'm not sure about the gradual increase and decline though.
The effect would be clearer to the player if it was instant.

This would balance war weariness nicely. War weariness gradually gets worse and instantly goes away, this effect would instantly happen and then gradually go away.

 

[Che Guava]

But should the player or AI treat every cancellation of a trade deal as a semi-declaration war the next turn?
If not, then you haven't really changed much.

WHat it would do would put a little bit of brakes on a total foreign policy turnarourd. If you don't think it's a big enough change, then let's say that the penalty applies up until 5 turns after a cancellation of deals, to provide at least a little distance and a 'pretext' for an invasion.

This is Civilization, not Kumbaya-Hippieville

Of course not, but in history plenty of empires use petty excuses for major wars, simply because it justifies thier position ever-so-slightly, and keep thier international reputation intact.

And I don't think we want to get into developnig a specific cassus belli system.

Sez you...

Nothnig like these exists in Civ5. I take your point, but I still think that we don't want the player to feel like they're having to fight against the diplomatic preferences of their people.

Who said anything about thier people, I'm talking about thier repuation with other civs! If you, as germany, attack france suddenly and without warning, your people aren't upset, but relations with Russia, Britain, Italy, etc might cool a little because they know that you can't be trusted.

 

[Ahriman]

If you are hostile towards a civ, your people will over time develop a hatred for them. If you are friendly with them, over time your people will begin to love them. The important thing is that their attitudes change slowly, and if you make decisions that go massively against popular opinion you will see riots etc.

But what are these hostile and friendly actions? There are a limited number of diplomatic actions to take in Civ. How can you have gradual chances with so few actions?

Of course not, but in history plenty of empires use petty excuses for major wars, simply because it justifies thier position ever-so-slightly, and keep thier international reputation intact.

But we have no system of petty excuses in the game, and its not worth modeling one. Much easier to let players declare war whenever they want.

Who said anything about thier people, I'm talking about thier repuation with other civs! If you, as germany, attack france suddenly and without warning, your people aren't upset, but relations with Russia, Britain, Italy, etc might cool a little because they know that you can't be trusted.

Not quite... you were talking about Germany attacking France suddenly *when France was your friend*. [We don't want to discourage people from declaring war on their hated enemies!]

To define "be your friend" it has to include something about what your people think about France.

That was your whole idea; to make it more difficult to attack a country that had been friendly towards you.

 

[Che Guava]

But we have no system of petty excuses in the game, and its not worth modeling one. Much easier to let players declare war whenever they want.

Hey, I gave you three examples of 'excuses' above that already exist in the game, and all you would have to do would be to attach them to a reputation modifier.

On top of that, civs, AI or human, will ALWAYS attack when they want to, this just makes it slightly more advatageous to attack at one time, when your repuation is less likely to be damaged and your ability to keep friendly with other civs while at war is enhanced. There is no requirement for a casus belli, but a particular event could precipitate a war, if the player chooses to. Personally, I think it would add another dimension to declarations of war that would be both interesting and historically relevant.

Not quite... you were talking about Germany attacking France suddenly *when France was your friend*. [We don't want to discourage people from declaring war on their hated enemies!]

[...]

That was your whole idea; to make it more difficult to attack a country that had been friendly towards you.

Yes, but that has nothing to do with how you are perceived domestically, it just makes being an international villain less desirable because other civs will recognize that you are untrustworthy. The 'friendliness' element is just how much that other civ has invested in thier friendship with you.

 

[Dale]

Much easier to let players declare war whenever they want.

So why don't you want to allow the AI to do that? Why accuse the AI of cheating, and then allow the Human to cheat instead in the proposed solution? Do you want the AI to play like a realistic human or a limited AI? Now you've gotten me all confused.

 

[Ahriman]

Hey, I gave you three examples of 'excuses' above that already exist in the game, and all you would have to do would be to attach them to a reputation modifier.

Events are very rare, and won't apply in mods. Espionage is gone. If you can only declare war if someone trades with someone with whom you're at war... how do you start the first war?

Yes, but that has nothing to do with how you are perceived domestically

Never said it did. It has to do with how your domestic populace perceives the world though.

So why don't you want to allow the AI to do that? Why accuse the AI of cheating, and then allow the Human to cheat instead in the proposed solution? Do you want the AI to play like a realistic human or a limited AI? Now you've gotten me all confused.

Sorry, when did I accuse the AI of cheating?

What I prefer is a system mostly like Civ4, where the human player can declare war whenever they want, but AIs are much more likely to declare war on enemies than on friends.

Is this an advantage for the human player? Sure. But it is simple and makes the game more fun. It makes the AIs feel more like countries than Deathmatch Bots, but still allows for full player agency, where the player doesn't have to fight against the will of their own populace (a la Civ1 Democracy declaration of war failures).
This is how its nearly always worked in Civ (early exception for democracy/republic), and in the vast majority of strategy games with diplomacy engines.

So my preference is for the AI to play like a limited AI, and then crank up the difficulty level so you are still challenged.

But here we're working on an alternate hybrid system that drops to bilateral diplomatic relationships (Factions A and B have diplomacy value +X) rather than a unilateral system (Faction A has diplomacy value +y with B, Faction B has diplomacy value +z with A) in order to make it simpler to define "friends" and create a penalty for attacking your friends.
This would reduce the level of "player exceptionalism" that some people seem to object to by penalizing the human for attacking friends, and would add in-game justifications for the desired (by me anyway) behavior where AIs don't tend to declare war on their good friends.

 

[Chalks]

But what are these hostile and friendly actions? There are a limited number of diplomatic actions to take in Civ. How can you have gradual chances with so few actions?

Well, for an extreme example, a mutual protection pact lasting many turns ending instantly with a declaration of war.

For a gradual transition, the mutual protection pact is terminated resulting in peace for a few turns. Extortionate demands are made resulting in refusal, further souring the reputation. Trade embargo, closing of borders then war over a period of many turns.

If you can qualify "ruthless backstabbing" and "gradual degeneration of relations" as the two extremes, then it boils down to defining the tolerances of the mechanic for gameplay balance.

Basically, the opponent (and your civilization's own people) should be made aware of your hostility for a certain amount of time to avoid the "back-stab" issues

 

[Che Guava]

Events are very rare, and won't apply in mods.

Not so rare, and besides we're speculating as to how ciV will work anyhow, mods or none. And since we're basically talking about a suggestion/modification anyhow, there's nothing to say that they couldn't be more common.

Espionage is gone.

And will be missed , but again, there's nothing to say that events couldn't have a an espionage theme to them, with consequences.

If you can only declare war if someone trades with someone with whom you're at war... how do you start the first war?

Again, you don't need to have a casus belli to start a war, it would only effect the consequences of going to war, weighed against a myriad of other factors. Inevitably, just as in history, wars will happen anyhow, as many civs sim ply won't careabout thier reputation, if they are strong enough.

Never said it did. It has to do with how your domestic populace perceives the world though.

No, it really doesn't. A leader cares, because other leaders will look on them unfavourably if they are underhanded. Nothing even slightly domestic comes into play.

But here we're working on an alternate hybrid system that drops to bilateral diplomatic relationships (Factions A and B have diplomacy value +X) rather than a unilateral system (Faction A has diplomacy value +y with B, Faction B has diplomacy value +z with A) in order to make it simpler to define "friends" and create a penalty for attacking your friends.
This would reduce the level of "player exceptionalism" that some people seem to object to by penalizing the human for attacking friends, and would add in-game justifications for the desired (by me anyway) behavior where AIs don't tend to declare war on their good friends.

So under this scenario, what are they penalties for attacking a friend? Domestic unhappiness? DIfficulty in making friends with other civs?

 

[Ahriman]

Not so rare, and besides we're speculating as to how ciV will work anyhow, mods or none. And since we're basically talking about a suggestion/modification anyhow, there's nothing to say that they couldn't be more common.

I don't want to be bombarded with events giving me cassus belli against various different factions.

And we don't want to design the diplomacy engine (a core mechanic) to be based around setting-specific events (an optional mechanic - many players turn them off for example).

And will be missed

Not by most. Many, many people think it was poorly implemented, with little game impact and annoying MM requirements.

Again, you don't need to have a casus belli to start a war, it would only effect the consequences of going to war, weighed against a myriad of other factors. Inevitably, just as in history, wars will happen anyhow, as many civs sim ply won't careabout thier reputation, if they are strong enough.

Why should I need a specific event or whatever to trigger in order to attack my hated enemy without diplomacy penalties vs every other civ?
Haven't we got mechanics that already deal with this, through penalties for attacking the friends of other factions?
If all of Europe hates the Arabs, then I shouldn't lose reputation with the other European powers for starting a crusade against them without some particular cassus belli event having occurred.

No, it really doesn't. A leader cares, because other leaders will look on them unfavourably if they are underhanded. Nothing even slightly domestic comes into play.

So how do you define "underhanded" for the human player?
Not every declaration of war is "underhanded".

So under this scenario, what are they penalties for attacking a friend? Domestic unhappiness? DIfficulty in making friends with other civs?

Potentially either of these.

 

[Che Guava]

I don't want to be bombarded with events giving me cassus belli against various different factions.

And we don't want to design the diplomacy engine (a core mechanic) to be based around setting-specific events (an optional mechanic - many players turn them off for example).

"We"....?

Personally, I like random elements in gameplay. I find gameplay can get a little tedious and pretictable without them. But to each thier own. But ok, let's leave that aside and apply it to another new aspect of the game: city states.

If you are on friendly terms with a city state, receiving bonues or tribute from them, and they are attacked by another civ, that is what could be considered a justifiable reason to go to war, and your reputation with other civs would remain as it is. Starting a war for no reason at all (just for pure territorial gain) would be 'unjustified', and other civs would henceforth be wary of your intentions.

Similarly, if another civ asks you to come to thier defense in a war, that is also 'justifiable': ostensibly you are there to defend them, not to grab something for yourself, even if that is your real reason to go to war.

Keep in mind that there is nothing at all stopping you from starting an 'unjustifiable war': you will just suffer a bit of a loss of reputation as a result.

Why should I need a specific event or whatever to trigger in order to attack my hated enemy without diplomacy penalties vs every other civ?
Haven't we got mechanics that already deal with this, through penalties for attacking the friends of other factions?
If all of Europe hates the Arabs, then I shouldn't lose reputation with the other European powers for starting a crusade against them without some particular cassus belli event having occurred.

If you're already thick as thieves with the rest of europe, then overall, it probably shouldn't affect your standing with them too much, and if they decide to jump in on the war on your side, the diplomatic bonus you get from being war allies will overshadow any loss of reputation you might have.

China, on the other hand, who was neutral to both the middle east and europe, might see you a little differently, and decide to cancel thier fus order and trade thier silks for incense with the Malinese instead.

So how do you define "underhanded" for the human player?
Not every declaration of war is "underhanded".

Well, besides the justifiable/unjustifiable scenarios described above with city states, let's say that, for eg, you're fighting a war with Russia and France comes to your aid. A turn later you make peace with Russia and turn your guns on France, because they've sent thier armies to Russia. That's a dick move! And since all the other AI's know that your supposed 'hypothetical' relations are good ("You came to our aid during war/+5"), they recognize that you are a dick. And your reputation suffers accordingly.

Domestic unhappiness? DIfficulty in making friends with other civs?

Potentially either of these.

But that is exactly what I want to have: a system whereby it is difficult to make new allies if you attack your former ones. Leaving aside any of the cassis stuff for the moment, I really do think that this would be best served by having some kind of modifier tally for all players, counted by all AI civs (since they've had contact with you at least) that gives them an indication of your reputation.

If you are loyal to allies (heed thier calls to war), share your technologies with smaller civs (or do whatever it is that is happening in ciV) and resist the urge to attack your neighbours unprovoked, you rack up 'Repuation Points'. With higher RPs, all (AI) civs will be slightly more inclined to have better relations with you because everyone knows that you treat allies well. On the other hand, if you deny requests for aid, turn on your allies, start unprovoked wars and act like a dick, your neighbours will have cooler relations with you, and might take an opportunity to wage a war on you in a moment of weakness, because they'd expect you to do the same thing.