Diplomacy is based on many factors but in short it works as follows:
From the point of view of campaign difficulty level:
VH level – AI considers player as its biggest threat so it tries to form alliances against the player and tends to go to war against the player. The AI factions easily form alliances if they consider the player their biggest threat.
H level – AI considers the player as a threat and tries to form alliances against the player and tends to go to war against the player, but to a more limited extent than on VH level. If an AI faction neighbors a faction that is significantly stronger, it will try to find an alliance against such faction. In such situation AI may propose alliance to the player and be a loyal ally.
N level – AI tries to form alliances against the strong empires (factions that are significantly stronger than AI) and tends to weaken them. If there are no such strong factions, AI will try to go to war against the player. If AI does not border the player, it tries to attack its weakest neighbor (if it tried to attack the weakest neighbor first, the campaign would be rather boring ).
In general:
If the AI’s invasion priorities (i.e. code command driving AI to invade a given target) do not exceed a minimum threshold, the AI will not attack other factions (except for rebel / neutral provinces).
By using the „opportunistic invasion” command in the options of war and rebel provinces conquer, AI will sometimes ignore its diplomatic priorities. Besides, M2TW AI is bugged anyway.
Alliances – if an alliance is not aimed against an enemy which is the AI priority target, it works as a „non-aggression pact”. The alliance code includes commands which drive AI to attack a common enemy but if it is not the AI perceived “priority enemy”, the chance that such attack will be executed is rather small (especially on VH difficulty).
Preparations for war
The tactics responsible for invasion is rather a slow and “lazy” procedure so the AI will usually gather its armies along the borders before it finally decides to invade. Also AI will train its best available units and agents – do not be surprised when an AI assassin kills your favourite general. For the human player this knowledge is not a big facilitation though, since the fact that AI is gathering its forces along the border does not mean that it is going to invade the player’s faction in the nearest future.
In some circumstances (for instance when the player does not wage a war with its neighbors), a neutral AI faction will also keep its garrisons on the border, just as it waged a war against the (neutral) player. This AI behavior may change when the it is in war with other factions – in such case it may send all its forces to the endangered front.
War
The highest priority of AI is besieging settlements – AI uses almost all its military potential on a given front to besiege settlements. It is probably not the best solution but taking into account the limitations of the M2TW engine it seems to be pragmatic and sufficient – otherwise AI would wander its armies aimlessly over the map.
Then, as a second priority, AI tries to attack the available targets in range, so sometimes when besieging a settlement, another AI army may come to support the siege or even attack a defending player’s army which stands near the besieged settlement (to help defend it in case of assault), while the first AI army still keeps the siege.
Finally, AI tries to do an “opportunistic attack”, which in some circumstances may ignore other AI settings and priorities. But thanks to this “opportunistic” possibility, AI is more “intelligent” and able to use some unexpected situations to its benefit. Since AI tries to besiege settlements, sometimes it may happen that it will spare player’s army standing on the bridge / ford and will walk around to lay a siege to a settlement or spare the closest strongly garrisoned settlement to lay a siege to the weaker one.
- AI trains the troops of good quality, generally better than in the majority of mods I’ve seen and I think even better than in the original Bellum since the troops quality depends mainly on the tactics settings in file descr_campaign_ai_db – I don’t know whether the twc modders know that.
- The additional advantage of such tactics is the AI ability of starving the besieged settlements – AI will not attack until it is certain that it will win (taking into account the result of the autocalc battle).
- AI is able to hire the mercenaries, which I did not observe frequently using other M2TW CAIs,
- Of course AI can spam agents in some circumstances,
- When AI declares a war, it can – especially when the border is long – attack simultaneously with several armies in different places along the border, which can demolish a careless player (I hope you don’t use the unhistorical “reload” ;P ).
- AI uses its agents better,
- AI can successfully strike with a really good sea invasion. This part of the code is perhaps not perfect but in my view it is quite good in comparison to other M2TW CAIs - I would like to see a CAI code which does it better.
Defence:
- Here I coded the AI in such a way that it is not so easy to conquer the settlements by attacking an army standing near the settlement and wiping out the settlement garrison that joins the battle as reinforcements (AI tries to place its armies a bit farther from settlements).
If the total strength ratio on a given front is unfavorable to AI, it will not attack openly but will rather sit in garrisons or try to defend based on bridges / fords and prepare ambushes in forests. On some occasions AI fooled me with the following scheme: I see a weak army near the forest, attack, the AI withdraws to the forest, I attack… and then I am ambushed by significantly stronger AI army. This is not very frequent but the AI in the old M2TW can show some tricks – it’s a pity that the original authors made so weak use of the AI potential in the vanilla game.
Even on N difficulty if an AI faction concludes that it no longer have common interests with the player and at the same time the AI faction is not in war against any of its neighbours, it may break the alliance with the player. In such case only relations 10/10 or 9/10 can protect the player from breaking the alliance by the AI. If AI is in war with a non-neighbouring faction, it does not count as a war and AI may break the alliance as soon as its common interests with the player end.