So fight against other humans and via a suprise invasion when they are fighting off another race. That risks becoming an enemy or at least an untrusted state to other human countries, which are the majority as far as I remember. It would have dire consequences on trade and other relations and would always serve as a nice justification for anyone looking to rile up support against them. No telling how relations with their supposed new ally would turn out either, they might just turn on them the second they feel strong enough, if they have such a big bone to pick with humans.
Even if you ignore all that, waging war on half the continent in the vague hopes that it might put you in the elves' good graces is just nonsense. At this point you;re better off just using the same resources to fight off the monsters, at least the result is clear.
