If we could all agree to go along with who the leader is, then we're all living in the same society.

But that's not the point: the point is "who's going to make me?" when our disagreements are significant.

And significant is minor details like "I believe I own this land" and "I have a right to exist".

EDIT: it's also worth noting that we resolve wars by simulation all the time - that's what diplomacy and it's supporting institutions are. Both sides present their objectives, tend to outline their military forces and escalation priorities to each other, and then go away and decide if they believe the outcome is substantially worse then what could be achieved by negotiation - which with war it usually is.