> Do you think Trump was referring to an agreement that was in place prior to the war?

Either this, or else: "agreeing to a point of contention" simply means agreeing that it is a point of contention, not agreeing about how it should be resolved.

> If so, why did the war happen at all?

Because of some combination of:

* there were new points of contention;

* some few unresolved old points of contention became more salient.