On top of that, I would argue that war was able to be made illegal because the major world powers found war among themselves to be incredibly ineffective. Many

On top of that, I would argue that war was able to be made illegal because the major world powers found war among themselves to be incredibly ineffective. Many years, massive amounts of money, and hundreds of millions of working/reproducing aged citizens died for almost no gain by most countries. It makes a lot more sense to work things out through international policies and economics than by bombs and guns. All that being said, Russia still invades Crimea and Georgia, maybe because it's just easier and faster with absolute overwhelming force.