Initially, what brought the 13 colonies together was the fight against a common enemy, the English Government. Since then, abroad wars is what has proven the best way to keep national unity. I'm thinking World War I and II, the Cold War, the Vietnam War and the current wars in the Middle East. Abroad wars keep the enemy outside and the fight in other lands, preventing Americans to turn against each other in a considerable scale. When Americans turned against each other it almost caused the division of the nation, during the Civil War.
I think this question may have many answers, this is just my opinion.