If there is a conflict between two sides and one is weaker, rather than accepting defeat, it'll strengthen itself with any means at their disposal...
One side will be the aggressor, should the other side let themselves be occupied or "assimilated" for the "greater good"? if there is a huge difference in power, they will(nowadays at least).
While greed exists, so will war, but the absence of greed doesn't mean the end of war.
Is it greedy for a mother to want her child to have water/food instead of you?
All of the above applies to conflict in general, not just all-out state wars.
Edit: Don't forget economic warfare, one could argue that economic warfare is happening as we speak, aside from all the civil wars.