I haven't played CivIV since before the 2.13 patch came out. I quit playing because the AI were too friendly. I always enabled the aggressive AI, and played small-medium maps crammed with lots of AI in it hoping to have wars early on to fight for territory. Only once did I ever see an AI civ wipe out another one.
The last game I remember playing, just as an experiment I purposely did not build many military units. I only had the conquest, cultural and domination victories enabled. I only kept one defensive unit in each city that bordered another civ's territory. I had massive cultural influence and lots of happy citizens, but I was the weakest in terms of military strength.
I made it all the way into the modern era before an AI civ declared war on me. The AI did have a few small wars with each other, but nothing major, each only lasted a few turns, and from what I could tell all they did was cross the other's border and destroy a few tile improvements. They would try to get me to help, and I would refuse, which is probably the reason I was eventually attacked.
As soon as the AI declared war on me, I switched all cities to producing military units. I was somewhat impressed by the AI's tactics, he had his military units staged outside of my borders in the "fog," and he declared war on me and marched his army across my border and took out three cities in one turn. The AI was able to take over about of my territory before I built up a strong enough military to defend what I had left. As soon as I won a couple of battles, he came begging for peace.
I remember the AI being a lot more aggressive in Civ III. I remember one game where I was on a continent with two civ's on each side of me. They declared to war with each other. I stayed neutral, but did not have open borders with either one. If you never played Civ III, in Civ III you could cross the borders without declaring war, but you could contact them and tell them to leave and they had one turn to leave before their units were booted back to their territory. My territory completely separated them, so they either had to go through me or transport around me in the water. This was still fairly early in the game right around the start of the medieval period, so ships were still primitive. I kept forcing their units out of my territory, until they started offering me things for open borders. I ended up giving open borders to the highest offer.
It was fun watching that war while. Later on after they settled their differences, the civ that I refused to let through my territory declared war on me. That was actually the largest civ on the continent, but I was able to defend myself, and after a while he gave up.
The other continent had 4 civs on it, and they were constantly having battles, even wiping one civ out.