Did you examine my hypothetical scenarios? They all basically revolve around
another race that wants to commit xenocide
on us. Or can you simply not conceive of a scenario where humans are not the superior species calling the shots? If we are not the only sentient race, then there's bound to be quite a few out there, and I can guarantee you we aren't the best, the strongest, the most adept or adaptable, or the most competent. There's always someone better, and odds are if the other race finds us, they are superior (they managed to go exploring and expanding before us).
The history of human wars is purely an example demonstrating that even when there is enough land to go around, there are times when you have to fight (unless of course you are ambivalent towards your own survival). To use your apparently favorite example, Hitler, America engaged in WWII and yet did not win a scrap of land. Nor did America want land. Was this just some stupid unjustified, unreasonable mistake?