The US was never isolationist unless you ignore the existence of Native American Nations; it was brutally expansionist from day one. From the time of the Monroe Doctrine, US imperialism expanded even further, leaving the US “isolationist” in most of the 19th Century mainly only in regard to what happened outside the Western Hemisphere, and not even always there.
The expansion wasn't all of it; it was also an era of technological innovation in which the Americans were leaders (though Europe also produced a fair amount of innovation). But the expansionism had another advantage: while Europe was busy fighting a series of wars for control over the same territory, the US had a lot more freedom to devote to increasing production rather than destruction.
So the 19th century may not be an accurate model for the 21st. Isolationism was more feasible then because it was a large, self-sufficient nation. Today, capitalism has broken production down into finer and finer pieces and it's much harder for even a very large nation to compete against the combined strength of the rest of the world. If we don't collaborate, and others do, they'll gain a relative advantage that will slowly eat into our dominance. We can't simply conquer new territory because there isn't any, and even if we did, ownership of land isn't as important in a technological era.