How do you handle the situation where there are two opponents in an at war territory (already battled) and a third opponent comes in?
Does that new player decide on who he attacks? Or does he attack both players?
Page 9 of the English rules states the following:
"Q: I move into a Province ‘At War’ occupied by Legions from several opposing players. Whom do I battle? A: Choose one opponent and battle. There can be only one Battle per Province (or Sea) per turn."
So as you suggest the attacker chooses which of the opponents they will fight. After the battle the province will remain "at war" but the result of the battle may alter the military leadership track.