I know in past you guys were toying with the idea of reoccurring villains, and I think I have a solution for you. Every battle fought is the same war. Since the demons are agents of time than time itself should not effect them. They arn't being defeated every generation and coming back, but you are just deflecting an attack. This could explain why the enemies later on are harder to battle because your earlier generations were fighting the grunts or the first wave. Since they are attacking the world at the same time across the many time lines the enemy generals would have to be partially present at every major battle and can be defeated without running away yelling "I'll get you next time" but they could just kind of like laugh it off. This opens up the ability to have bonuses or handicaps depending on how well you defeated those generals in the past. And since these battles are all taking place at the same "time" across multiple generations there could be rips in time on the battlefields where you could see ghosts/visions of previous battles i.e. glimpses of your grandfather, future demons attacking soldiers, or even randomly dropped weapons or artifacts from the future or past can slip through the rips.