What will it take to unite America again?

During WWII, people drew together to support the troops by creating Victory Gardens and other shows of support. The Vietnam war tore America apart. 9/11 seemed to draw Americans together again. After 9/11, I saw American flags everywhere. Yards, cars, on houses, in offices. Churches in my area became packed. The only reason I can see for that is people wanting to draw together, to be with friends and neighbors. By 2003 or so, that all seemed to go by the way side. What do you think will draw us together again as a nation? Or do you think it will never happen again? If so, why?