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?
Straw man, argument 'self-defense == bad' not present.
Possible arguments:
1) The ethics (and humor) of pre-emptive strikes
2) Given that the proportion of resource-containing matter forming planets inhabited by sentient life to total resource-containing matter in the galaxy approaches insignificance, it seems extremely improbable that homo sapiens will ever be engaged in a competition for resources with sentient alien life at a time point where homo sapiens still exist and have not evolved into several other species.
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?