"In our obsession with antagonisms of the moment, we often forget how much unites all the members of humanity. Perhaps we need some outside, universal threat to make us recognize this common bond. I occasionally think how quickly our differences worldwide would vanish if we were facing an alien threat from outside this world. And yet, I ask you, is not an alien force already among us? What could be more alien to the universal aspirations of our peoples than war and the threat of war?"No, that wouldn't happen. That is optimistic to a Pollyannish level. Perhaps if there were massed invading ships so that there was no question that it was a hostile invading force, this would be so. Yet we have seen this throughout history, and human beings actually don't act that way.
The Romans hired outside tribes along the frontier to fight other invaders, and sometimes brought them to the center of Empire to fight their own internal struggles for power. Goths, Huns, Allemani, Franks, Vandals...and these are the very tribes that lead to their undoing. The leftover Romano-Britons brought in Saxons, Angles, and other tribes to help them in their fights against each other. Now the whole place is named Angle-land, England. Various Muslim tribes were happy to ally with the Crusaders against Seljuks or Sassanids they thought were more worrisome, and the Crusaders with Muslims. The Native tribes of New England tried to use their connections with the English settlers to push each other around, though some preferred to ally with the Dutch or French, and thus, eventually, the French & Indian War was inevitable. Later natives in the Central Plains and westward were happy to use the expanding Americans against the dreaded Comanches. Now all those tribes identify together and wish they had made a unified stand early on. The Romans eventually came to that conclusion as well.
Arriving aliens might arrive for trade, or exploration, or as some raiding party. Wherever they landed first would form a relationship with them and be perfectly happy to use them to their advantage against Terrestrial enemies.