Agree with Comrade Alexo. This game doesn't offer multiple paths to victory, it offers one - wiping out everyone in your way until you have enough provinces to trip the 'I win' switch.

It's important to see alliances in the game for what they are - temporary 'fight together' pacts between competing factions. In a modern context, it's more like a loose cartel agreement between businesses than a permanent merger. Both sides pause to allow them to compete more effectively against other threats, all the while watching their back in paranoia.

Another good comparison someone suggested was the pact between Hitler's Germany and Stalin's Russia. They were allies too until 1941, when Hitler invaded Russia causing Stalin to make the same sort of complaint as many members of this forum have made - "Hey, my totally untrustworthy backstabbing ally has just invaded me! The swine!". At least we forumites aren't executing the spies who brought us the bad news because we didn't want to believe it. Although I think some might if the game gave us the option...

If the AI respects alliances for as much as 2-3 turns, that's great. You can't leave a skeleton garrison in any settlement bordering an AI ally unless you KNOW that their armies are elsewhere and there's an easier non-allied target they can be going for. Otherwise, prepare to lose an ally and maybe a city/castle and some troops, because you've done the equivalent of plonking a large and open bottle of brandy down on the table in front of a recovering alcoholic and walking out. Will he go for it? What do you think?