Quote Originally Posted by WhiskeyGhost
like i said, i think it doesn't work because the AI doesn't leave mobile armies out with movement points left to do it. Also, yes, it did change the odds enough to win the battle w/auto-calc in the same perverse way that auto-calc thinks 4 trebuchets can win against a unit of knights.
Interesting... from my experience, the AI ally lost anyway, even when supported by my troops, which has led me to suspect that this option is actually broken at 2 levels, first in your not getting assistance from your allies (there should at least be some window popping up to inform you that an ally's nearby stack will march in to support you in your upcoming battle, or, the AI gets the same option that you do and always opts for not lending aid) and second the assistance not having any effect on the outcome of the battle (in sea battles it's more obvious, although I've never seen a popup-window in those cases, just "normal" battles where your ally is the agressor and your fleet just happens to be in an adjacent tile). Your theory about the AI not having movement points left sounds plausible, only that IIRC I got that popup window too in situations where my stack didn't have MP left and it still marched to the battle location.

Quote Originally Posted by abdecken5
the problem is that diplomacy is surposed to mean you dont have to defend your boarders from allies to fear them into not attacking you. The idea of diplomacy is that you neogociate before someone sends an armys onto your doorstep. Its a bit late to only open neogociations once your enemy is committed to attack you
I don't want to defend the AI programming, but you have to consider that it's quite hard to program the AI to have an idea about the big picture. The way the diplomacy and AI stratmap behaviour are intended to work is that an alliance with good reputation and good relations usually means that you can leave your border settlements relatively lightly garrisoned. But if relations are not so well, the AI will try to take a settlement from you that's defended by just two units or so. It will do that because it estimates the settlement will be an easy picking and it does not think ahead and realise that such a maneuver will not sit lightly with you, moreover, it doesn't take into account the reputation drop it will suffer through backstabbing and lastly, it has a warped picture of your military capabilities since it can only judge your strength by auto-calc ratios which are indeed the root of many bad military decisions by the AI.
So the bottom line is that diplomacy alone is not sufficient to guarantee a lasting alliance, but without paying attention to diplomatic relations, you will have even more unnecessary wars on your hands. In addition to that, larger garrisons really also decrease the chances of an ally besieging your settlements. The only events that you cannot counter or avoid in any ways known to me are random port blockades and the like, which I think happen because the mission generator works for the AI factions too and doesn't take any alliances into account.
I've owned this game since last Xmas and it has taken me more than half a year understanding the diplomacy mechanics and learning how to use them in my favour, so if you're frustrated with the diplomacy (just like I was at the beginning), just keep playing and trying things out, watching the effects. With due experience you can minimise, though not completely eliminate, "stupid" AI diplomacy behaviour in non-blitzing campaigns.