I think I noticed this happen to me in CIV3 before, but this time while playing CIV4, it was almost too good. In my last game on noble level, Montezuma was my neighbor to the east. My relations with him were pretty bad most of the game. I finally had enough of him so I began stagging troops on my border in preparation to attack him. Closely looking at his military, I knew it was not going to be easy and I was a bit worried. So I wanted to do this the right way. Here I was, stagging my troops on the border, just after I had a threatening buildup, he declares war on me and attacks first. So he got the first punch in. He caught me on that road in the open just before I was to declare war, get to that hill and fortify my attack force adjacent to his city. I was not expecting him to do this and kind of had my guard down. It hurt as he stood toe to toe against my force that I thought was formitable, beating them back. I lost 80% of my main force and was in big trouble. For whatever reason, alot of my samuris were just not standing up against his horse archers, crossbowmen and elephants. He made his way to some of my weakly defended but important cities, took 2 and burned another down. He pushed further in, and within a few turns took my capital! Shortly after down goes another one of my cities. What a disaster! This was completely obvious that the AI noticed a buildup of troops near the border, got concerned and attacked me. By him attacking me first, it completely backfired on me. So is this something that does indeed happen? Building up troops near the border can cause a possible attack from the AI? If so, it is realistic. I think it maybe ok to position a couple of troops, but it could be when an attack force with 10 or more units are positioning on the border, its obvious that something is about to happen. Especially if your relations is bad with them. I have had the reverse happen to me, when the AI built up troops on my border and I attacked him before he could attack me. In the end, this war would have been totally different if I would have been able to attack him first like planned.