"Good morning. In less than an hour, aircraft from here will join others from around the world. And you will be launching the largest aerial battle in the history of mankind. Mankind ... that word should have new meaning for all of us today. We can't be consumed by our petty differences anymore. We will be united in our common interests."
That was, of course, Bill Pullman as President Thomas J. Whitmore in Independence Day. What does it take to unify humanity and let us get past our "petty differences"? A devastating invasion by space monsters. The same thing that got Captain America and Tony Stark to work together in The Avengers. The same thing that got the various warring clans of Scotland to band together behind William Wallace in Braveheart. It's right there, in all of our most popular movies: a war against unspeakable, inhuman evil is the only thing that makes us unite and put our differences aside.
"If it wasn't for the aliens, I totally would have dropped the N-word by now."
And if that singular, despicable monster doesn't exist, we'll create it. We have to.
Or, to quote another leader who rallied his people when things looked their darkest:
"As a whole, and at all times, the efficiency of the truly national leader consists primarily in preventing the division of the attention of a people, and always in concentrating it on a single enemy. The more uniformly the fighting will of a people is put into action, the greater will be the magnetic force of the movement."