It hasn't ended yet. Not all empires end like Austria-Hungary, the British Empire or the Soviet Union, with a sudden geographical disintegration.
Discussion
More likely Roman empire style death
Yeah, that's what I think. The US, like Rome, is mainly a commercial empire. The internal market aspect typical of 18-19th century empires was accomplished with the Manifest Destiny doctrine, the conquest/purchase of Spanish, French, Mexican and Russian possessions, and the Atlantic-Pacific railroad.