If aliens invaded, if they had the tech to get here from deep space then I think they would wipe us out united or not
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If we killed everyone and started again under one rule it still wouldn't work, people would rebel and try from their own clans or nations. It is human nature to fight maybe in thousands of years because of how tame we have become that instict could be over written, or maybe their could be a drug or something to supress it.