I kind of hate how the AI drops all sence and goes for backstab when it comes...

I made alliance with Date clan before the realm divider. I pretty much saved them from Uesugi and 3 other clans they where in war with. They where on huge defencive and had 2 provinces left. I married their daughter to my family. I gave them money... And then they declared war to me; they have one stack of units and two provinces. I have 20+ provinces and about 4 stacks fighting in each corner... This is anoying; why would a clan backstab its savior, allied, family and faction that will crush whole clan in few turns. It doesnt matter much in tactical sence; I'm holding half of the map and fighting the other half. But it does brake the illusion that theres I in AI.