The part of U.S. American History usually taught as the isolationist period has, in actuality, more in common with imperialism. An isolationist country is one that favors domestic policy at the expense of foreign affairs. Generally speaking, one would leave other countries alone, and expect to be left alone in turn.
Common arguments for proving that the U.S. was isolationist include its staying out of WWI and the League of Nations. However, these actions are not necessarily isolationist, as neither the war nor the league would have assured a benefit to foreign affairs. Regardless, the U.S. foriegn policy is marked much more by imperialist characteristics.
Notice for instance, that between 1776 and 1945 the country grew 600 percent. Seemingly, the U.S. came by a continent of free land, but of course, that is ridiculous. So why do historians consider committing genocide on Native Americans a form of domestic growth rather than external expansion? Simple, it's called Manifest Destiny, or the widespread belief that God gave the Americans all the land between the Pacific and Atlantic. Even as an Yank, I find this similar to the delusion that al Qaeda's mission was given by God.
If the U.S. was an isolationist country, that only wanted to be left alone, then how did its soldiers end up killing 250,000 Filipino citizens? I don't mean to simply bring out all of the atrocities, but the Philippines are nowhere near the land mass that God supposedly ordained. The U.S. occupied them as an outpost on the rest of the world, an imperialist method rather than an isolationist. How did it get gunboats into China? Why did it intervene 180 times in Latin America? How did it fight the War of Intervention, the Quasi War, or the War of 1812, all of which were with nations on the other side of the Atlantic? The list goes on.
Truth be told, the U.S. has never been an isolationist country. The Marines' Hymn actually begins with the words, "From the Halls of Montezuma, To the Shores of Tripoli." Montezuma and Tripoli are not located anywhere near the original 13 colonies. Tripoli, in fact, is almost on the other side of the world. The only foreign countries that it isolated itself from are the ones that could challenge us in any significant way. Thus, the Monroe Doctrine. This was the policy that created a theoretical divide between the two hemispheres to prevent America's challengers, Europe, from colonizing the other Americas. (Colonization was the key to power at this time.)
Another effect of this was that the U.S. would be alone with malleable countries, including Latin America and, not coincidentally, the Halls of Montezuma. Because of this, the Monroe doctrine can be seen as selectively aggressive, not isolationist policy. True isolationist policy refers directly to staying out of foreign affairs, and so is not a term that can describe this period of U.S. history.
I don't pretend that I know everything. Truth is subjective to perception. So, if anyone has another viewpoint, please tell me about it. Comments are very welcome.