I'm amazed that everyone has assumed that America was 'isolated'; she only isolated herself from Europe, to prevent what George Washington termed "foreign entanglements".
True she was very focused and determined on expanding Westward further into the continent, however in the period mentioned she went to war with Spain, most notably in Cuba, and then had a protracted war in the Phillipines, hardly keeping to herself!
This was all down to Teddy Roosevelt's "Big Stick" strategy, which led to the US acquiring Panama and the Philippines, and engaging in the ‘Banana Wars’ which she engaged in to protect her commercial interests in Central America.
So my argument is that America isolated herself from Europe as she didn't have a sufficiently powerful Military to interfere in any of their affairs without being severly damaged by it; instead she built up a significant area of influence in the Carribbean, Latin America, and the Pacific.