The Chronomancer AD&D supplement contained a model based on "historical inertia". The idea was that things that happened were going to happen pretty much regardless of what time travelers did. So if you were to go back in time and kill Hitler, the course of events would elevate another genocidal leader and the result would be almost indistinguishable. However, on a personal level, great changes could be made, you could rescue your sister from the concentration camp with few consequences, or get back at that asshole Hitler for having her killed in the first place.