This is the classic argument for appeasement. An aggressor invades a neighbor, kills a bunch of people, and takes a bunch of land, and the response is "Okay, stop killing people and you can keep that land if you promise really hard not to invade any more." In what world would that work?
All that does is broadcast to the aggressor and the world that invasion works. If you want a piece of land and all of the people and resources on it, just move in. It gives incentive to committing maximum damage and to grabbing as much land as you can because it will make future 'negotiations' go that much more smoothly. I don't think you really need me to provide any historical evidence of this happening, but I can. Anyone can.