Again, excellent points, but don't forget the most important resource Columbus and his band of marauders found: humans. Not only a plentiful resource for agricultural labor and domestic help, but recall that the Spanish (particularly Columbus's brother) abducted huge numbers of women and especially young girls and either sold them into sex slavery or used them that way themselves.
2) They need rersources and can only get them over our dead bodies. This part I CAN call bullshit on. Here's why:
What resouce do they need? Precious metals? Water? Gasses? Heat energy? What?
Any of these things can be found in great abundance elsewhere. Asteroids, moons, nebulea. Basically they could be poaching any resource they need through un-inhabited space and never even have to interact with the locals on our little blue ball.