Capturing (but not killing, so he can't be replaced) the opposition's leader in battle often does wonders, as you can lock them up in the Tower and leave their faction leaderless and defeated, taking some of the resolve to fight out of the enemy. It doesn't guarantee that the enemy lay down arms immediately but it can reduce their support enough that, if they decide to keep fighting, one more victory can end them - especially when the war in general is a civil war where said enemy can't fall back to their own country to regroup over the years. So, fiddle with a battle I would guess. Maybe change things at the first Battle of St. Albans so that the Lancastrians aren't caught off-guard in the middle of the city, the two armies meet for a piecemeal battle and the Yorkists lose?
Just throwing ideas out, didn't really think very hard about it to be honest.