Depends on so many factors. Sometimes the external enemy doesn't unite people at all. Sometimes it becomes a race of who can suck up to them so when the war is over they can run things. Traitors and collaboration.

Also I am betting if there was a threat it would be over in hours. As they hit us with a million nukes while our governments spend the last few minutes wondering why we pointed out best gear at each other instead of up.

Or heck they could be creative and just block out our sun for a decade and use bunker busting bombs looking for infrared to kill off the stranglers. That way they have a nice non-nuclear wasteland to work with.