IMO I don’t think humanity would unite. Some people would force their families to surrender to the aliens because they fear for their own lives, others would try to bargain with them for technology. Others would help the aliens simply because they hate humanity. We really can’t count on anything to unite us, and I don’t even think uniting the species should be our common goal. People are too individualistic and diverse for that, and unity would take away what it means to be human.