I know this sounds weird, but American needs an enemy. We are "united" (pardon the pun) by a common foe. We are at our best when coming together in the face of opposition.
First, we had England. Then, it was the Indians. Then Mexico. Then Germany/Austria-Hungary. Then, Germany/Japan/Italy. Then the USSR. Then Iraq. Since the USSR, we really haven't had an enemy. We've had to manufacture them (yes, I think that AlQ is a manufactured enemy, as they really don't have the ability to threaten the US in a meaningful way).
Not saying we need to go starting wars. But, it sure helps bring us all together when there is a big, baddy out there.