A formal ally in a turn-based game informed me that he would quitting on the following turn, so I declared war (tick 832). When he conceded on the following turn (tick 840), the AI declared war on me. The game is now behaving as if the FA were broken on tick 840 instead of 832. It is now tick 864, and my vision from his stars just expired. Our carriers are still coexisting until they will (presumably) fight on tick 865.