Outgroups and colonialism

I'm currently reading The Birth of the Modern and the sections on colonialism in South East Asia.  It had always puzzled me just how that worked.  It's one thing for the Spanish to just come into the Caribbean and be able to dominate the inhabitants by main force.  But when expanding into state societies Europeans were usually outgunned until the mid 19th century, at least on land.  There's no way that Cortez could have conquered Mexico without local allies.  And you could say that same for European colonies through Asia.  The Pilgrims, in fact, wouldn't have survived with the explicit generosity of the Native Americans.

I think the best explanation from this is probably a thing Scott Alexander mentions in I Can Tolerate Anything Except the Outgroup:

 Freud spoke of the narcissism of small differences, saying that “it is precisely communities with adjoining territories, and related to each other in other ways as well, who are engaged in constant feuds and ridiculing each other”. Nazis and German Jews. Northern Irish Protestants and Northern Irish Catholics. Hutus and Tutsis. South African whites and South African blacks. Israeli Jews and Israeli Arabs. Anyone in the former Yugoslavia and anyone else in the former Yugoslavia.

So what makes an outgroup? Proximity plus small differences. If you want to know who someone in former Yugoslavia hates, don’t look at the Indonesians or the Zulus or the Tibetans or anyone else distant and exotic. Find the Yugoslavian ethnicity that lives closely intermingled with them and is most conspicuously similar to them, and chances are you’ll find the one who they have eight hundred years of seething hatred toward.

And of course when you see people around the world allying with Europeans because they hate or fear their neighbors.  Chief Massasoit of the Wampanoag through that the Pilgrims would be a useful buffer state between them and the Narraganset who had been causing them trouble and that's why he was willing to assist the Pilgrims.  For 50 years that worked out fairly well.  For the inhabitants of Mexico it went bad much more quickly.

Often in science fiction you'll see the world uniting in the face of alien invasion.  Reading about these episodes in history makes me very skeptical about that.  I don't think the US would invite aliens in right now just because the US is in such a secure position but I could easily imagine the US in during the Cold War trading large stretches in the South West for protection in the case of Soviet missile attack.