Empires are hard to maintain, probably impossible. From Doug Bandow at antiwar.com:
When the new American nation was created, it was a lightweight in an international political game dominated by heavyweights. The U.S. was forced to develop a serious, measured, and realistic foreign policy.
The colonists outlasted the British during the Revolution in part because the New World revolt triggered an Old World war, in which the United Kingdom also had to fight France and Spain, which allied with the colonies. London’s North American battle became secondary. Yet French and Spanish assistance for the colonists remained limited, intended only to weaken the UK. The absolute monarchies did not desire a strong, independent republic on a continent where Paris and Madrid still possessed colonies.
It was a dangerous world for the weak, young nation. Nevertheless, the purchase of Louisiana from France in 1803 removed one threat from the continent. America survived – barely – another military round with Britain in 1812. A decade later colonial revolts against Spain seemed to dispatch the last serious regional rival.
President James Monroe then announced in 1823 that European efforts to recapture old or conquer new colonies would be seen as exhibiting “the manifestation of an unfriendly disposition toward the United States.” At the same time, he publicly eschewed intervention in European affairs.
This was pure chutzpah, given America’s evident lack of a military capable of enforcing such sentiments. Nevertheless, the proclamation was a fine effort to bolster US security. Europe, the fount of war for centuries, should stay out of Washington’s neighborhood. And the US would not get entangled in the Old World’s endlessly disastrous conflicts.