In American mythology, mankind always stands together, unified under great need in fine fellow-feeling if aliens are coming. If they are coming with obvious hostile intent, we figure out some crafty but risky space weapon that just might work, and the penultimate scene is of the world's peoples - a wizened oriental guy, not in a robe but clutching some artifact like a colorful cloth and not actually praying, but y'know, it's the same thing; Actual Chinese scientists have to be involved, and we learn something from them and they from us through the whole experience*; black people and white people openly admiring each other's abilities in a crisis; a top woman scientist has to have a son or husband or Uncle Nelson in particular danger as the moment draws near. (But she holds up strongly, of course. She's a professional.) Arabs - you can tell by their kaffiyeh; various wise European men and beautiful women and smart-as-a-whip Africans; are all there. Indian scientists are present, you can tell by the dots (I don't care what it means! I want dots so that everyone knows they are from Pakistan or India or somewhere like that! Makeup!) No one is left out. Okay, maybe people with southern accents are left out, or are in the way, ruining everything.
Or, if it's one of those gradual contact stories, where someone discovers, almost too late, that they are really here to make us work in deadly styrofoam factories, it's still the same. Human beings stick together. Those damn Others are threatening our entire species dammit! Warn the Brazilians and Indonesians, man!
That's the American version, and we, plus a few others, might act that way. But history suggests not. If the reptiloids landed first in China, whether by accident or design, and had some technology which gave them an advantage over us, they just might pal up with the reptiloids. And we, now encountering rival reptiloids from the same planet who said they were different than those other guys, we might invite 'em in, just for protection. And cash. Years later, when we were all picking hot styrofoam off of our whimpering children's fingers we might say "we shoulda stuck together," but not then. And those reptiloid factions might still be rivals on a hundred planets, utterly sincere in their alliances each time.
That's what happened in the Crusades, in the slave trade, in the expansion of the Roman Empire, in the Bantu migrations, in both the European and Chinese colonisations. People come to trade and they have cool stuff and we want in on that, and cutting the tribe next door out. If they fuss, we join the newcomers - who may not even be true invaders - against our traditional rivals. Hell, if reptiloids could have helped us against Memorial High School we might have gone along, especially if increased access to babes were somewhere in the picture.
Remember who makes these movies and writes these books. Us
*Russkies, of course, in the older movies/books, and the spiritual figure is a rabbi