Do AIs react to troops gathering on their borders?

[Fine_Distinction]

I was never making an effort to hide my intention to attack an AI - they are not humans, after all ; - I would just normally gather stack of death on a tile next to their borders, sometimes for many turns.

Recently, I couple times I did it AI seemed to react - either upgrade obsolete units that he kept in his border city forever, or peace- vassal to much stronger civ, which I was not ready to attack.

Does anybody know from code diving or experience if AI reacts somehow to troop movement near its borders?

 

[Kaitzilla]

I'm pretty sure the AI doesn't react to border buildups during peace time.

On harder settings it might feel that way, but only because the AI LOVES to upgrade its troops as soon as they get a good military tech.

On Deity, AI upgrades cost 90% less than what a human pays, so the entire upgrade happens in a single turn sometimes.

During war, the AI WILL react once you get within 2 tiles of a city.
If with an army, they will usually whip defenders, but not always.

If you really want to rile them up, blockade their coastal cities with a navy and watch them pour an endless stream of boats out to try and break it.

 

[Fine_Distinction]

Thanks, Kaitzilla, thought it might be coincidence, although it was really strange. Pacal has an axeman and couple other weak unit in his border city for ages, which I checked using spies, than the turn after my SoD approached (during peace), he suddenly upgraded them to grenadiers! He had pacemen tech forever, it was around 1700 AD. Next turn, the turn I planned to attack, he peace-vassaled to another AI That was on Monarch.

 

[Swordnboard]

One other explanation is that an AI will only peace-vassal when they are below the world's average power rating, so if you militarize significantly it may push an AI over that threshold, making them want to peace-vassal.

AI also tend to militarize all at once rather than slowly (build military in all cities when they are thinking about war, then ignore it completely for a long time if they are in peaceful build mode), so your timing may have just been poor / unlucky.

 

[Bibor]

The AI will place its stack in the city nearest to the where wars start or end:
- if it captured Madrid last, the stack will probably stay in Madrid for some time.
- if it wants to attack India, it will place its stack closest to Indian borders.
- if it's not in war mode, it will try to place its stack in a city near the most likely aggressor.

With this kind of information, and proper positioning, you can wipe out the entire AI stack in a single turn.

When an AI gets a new tech it will upgrade a few units, and the turn after that it will try to mass upgrade if it has the income. So keep the AI finances low if possible, especially before a planned sneak attack (extort money, sell techs etc.)

 

[ArchGhost]

- if it's not in war mode, it will try to place its stack in a city near the most likely aggressor.

My observation has been that this is only true for a limited period of time, based on wherever it previously moved the stack (such as taking a city and then suing peace like you mentioned). Stacking on border prior to an attack seems to only happen in war planning mode, but I can't begin to confirm this. My last game I played Caesar stacked on me all game and only moved them away when plotting on someone else or when dragged in to a war immediately. AIs at peace seem to inevitably place their stacks back into their capitol...especially if they have very large empires. But Caesar's behavior seems to throw a wrench into that idea, as I was clearly the easiest target (1/5 his power and he can plot at Pleased)

Either way, any information you can get in tracking the AIs' stacks is key in dealing with them. Scouting is still very important beyond filling out your map!

 

[Bibor]

@ArchGhost in one of my most recent games, I had Zara plant his stack on a city near our border (wasn't his capital) and he wasn't plotting on anyone. It later moved its stack to a city bordering another AI (Gilgamesh). Since his third neighbor (Japan) was a vassal of Korea (oh the historical irony), I presume it was a defensive move. I still can't say i'm right, though, but it appears so.

In my most recent game, Roosevelt, who also wasn't plotting on anyone, split his stack between its capital and the city closest to me (had a disproportionately large amount of units in these 2 cities compared to other cities). Since he was constantly annoying me with espionage missions, I figured something's going on under the hood. His predictions were right, though, I attacked when I got to bombers