Open Borders

Pros - if you have a religion, you can get your missionaries in there to spread it far and wide (assuming no theocracy, of course). Also, you can scope out resources, cities and defenses for war so you're not going in blind.

Con - The other guy can scope out your resources, cities and defenses.
 
Open Borders generate trade routes, which over time can become very profitable, especially if you have a lot of coastal cities with harbors and custom house, I don't look at AI code and all that but I'd also assume a civ is less likely to goto war with you if your generating them 50gold/turn. They also increase relations with the civ you have them with.

The bad part is it decreases relationships with civs that don't like the civ your trading with, they may ask you to stop trading with them or yatta yatta yatta, it just makes them cranky.

I never consider open borders just unit passage, it's a trade pact to generate commerce between your two cities.
 
I almost always open borders ASAP for the trade routes and diplo modifiers. This is extremely important for some civs, esp. neighbors--you get only a brief window before those "shared borders" negatives and other possible negatives pile up--once you get to Annoyed or lower, you usually can't open borders with that civ ever again without some big +diplo modifier like shared religion.
 
Open Borders:

Pros:
Allows units access to foreign civ's territory, which may open up colonizable land, or shorter transport routes.

Allows use of missionaries and corporate executives in foreign civ's territory - very profitable.

Allows cities to form trade routes to foreign cities, giving very substantial boosts to commerce.

Improves diplomatic relations with foreign civ.

Allows you to scout out their territory for future attacks.

Cons:
May allow opponents access to unclaimed land previously blocked by your borders (only in early stages).

Diplomatic penalties between you and the foreign civ's enemies.

They may spread their corporations to you.

They could theoretically socut out your territory for invasion, but this is only an issue in multiplayer. The AI simply isn't smart enough to make any use of the information.

Barring in the early stages when I has unclaimed land sealed off by my borders, I always open them to all possible civs.
 
I never accept this, simply from brainwashing from old Civ versions where they would backstab you (TREACHERY!!) and yada yada. probably dumb, but it's just me. I don't share. :)
 
They could theoretically socut out your territory for invasion, but this is only an issue in multiplayer. The AI simply isn't smart enough to make any use of the information.

You know, you'd think this would be the case. But I've had it happen a few times (and a couple of times I had saved so I went back and replayed) and had the AI declare on me after a few rounds of OB.

Whether they make use of the information or not, when I went back and didn't agree there was no declaration. (and no, I didn't suddenly beef up military/increase power on the re-do)
 
I always take Open Borders as the Foreign trade is enticing. The only downside is on larger maps where you have to play a side so you keep getting asked to cancel your open borders.
 
I think if the AI can see that you have a lightly defended city on there borders they are more likely to attack actually.

I think I've seen it pop up on the little game tips and I def noticed it playing Aggressive AI, if I have a city with only a few defenders in it they have a much higher rate of attacking.

Open Borders would effect that by letting them see into your cities, where as they may or may not be able to see inside your city and what's defending it without one.
 
Yeah Sick, I think you're on to something.

Usually my border cities are very well-defended and my interior ones less so - I guess when I signed the OB in those cases the AI noticed that my empire was like the Wizard of Oz.
 
The Open Borders trade greatly increases the trade traffic. No mercantilism free specialists can achieve this.
Cons:
1.Extremely boring cons is the AI non-stop is trying to find a free tile in your back and settle a town there, no matter how bad an unusable the terrains is. 2. If you had an open borders with nation it usually send several units to explore your territory. If you start a war wit this nation (or if it starts a war with you) its units will be teleported to the neares free tile. It may happen this free tile ito be in the middle of your empire where you guard your cities with a single spearmen agains the enemy cavalry or even infantry.
 
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