Well, it's not as it the US is absolutely isolationist... there are resources from abroad that must be imported (particularly oil). To be sure, the US has a very strong 'self sufficiency' attitude, but there are some minerals and chemicals that simply can't be obtained here, things that are vital for a modern industrial society. So, I'm going to assume you meant the USA's infamous political isolationism.
To break that, you have to ask "what's in it for us?" Just why would we suddenly take an interest in meddling with the political situation around the world? All the USA really needs is to ensure that those imports keep coming in... and frankly, governments come and go, but they all need money, and none are going to cut their throats by embargoing us. Particularly since the USA has a firm reputation for paying cash and not interfering with whatever abominable local customs you have. To put in bluntly, the USA is too strong for anyone to attack conventionally; is there anything wrong with 'we don't bother you, so don't bother us'? If the USA suddenly started throwing its weight around the world, well, that would likely mean that we wouldn't have our balanced budgets, our vaunted health care system, and a lot of other things that we take for granted now.
So, to get the USA out of it's (extremely advantageous) isolationism, you have to have a damn good reason for us to do so. The one tiny possibility I can think of is if the USSR had been led by that guy Stalin, who had everyone worried for a few years. From all accounts, he was a rather nasty sort who wanted to carry out Communist insurgencies everywhere he could do it. So, let's imagine that he actually came to power and ruled for a couple of decades... that just might actually get the US out of it's isolationism, if only out of self interest. But even I have to admit that this is kind of a long shot...