You might think that eventually a war ends and someone ends up with more than they had before the war broke out so they gained right? Not exactly true.
You see war never really ends, as I'm sure many are aware. Once WWI ended the world spent their resources gearing up for WWII and once WWII ended the world spent their resources gearing up for WWIII. So it's a cycle of preparing for war, fighting war, repairing from war and preparing for war again. All the resources gained from a war are either spent on the war or preparing for the next war to be fought over the same resources. In the end it's pointless because the resources are only ever used to fight over the resources.
There is far more to be gained from sharing our resources with one another than from fighting to keep them for ourselves. The resources are lost in the fight and therefore nothing is gained, only lost. Only losers go to war.