I currently have a ceasefire going. I attacked an enemy home world, and said ceasefire friend came to help out...which is strange, given that we aren't allied, but whatever, I'll take the help. The weird part came after, when the planet was taken and colonized by me. Now, his fleet is just circling around in attack formation, but not attacking. I thought at first that the A.I. was just waiting for a good moment to attack. Since he has started, however, he has not left. Any window for attack is long since gone, as my main fleet is now present, along with an upgraded starbase and host of defenses. Here's a few pictures of the issue: