I disagree, the U.S.of A. is a very young country with little historical significance before the 20th century, it's only been of any real importance for about the last 70 years which is about three generations, which is a blink of the eye in terms of human history, and really the sun is already starting to set on that. Its dependency on Canada now for electricity, oil, and natural gas weakens the economic strength it's enjoyed since the collection of war debts from the two World Wars, and shatters its independence because it must have energy sources to survive. It's banks are falling apart, it's infrastructure is collapsing, it's housing market is a horrid mess, and it's manufacturing is leaving or going bankrupt, and that's just touching on the economic problems (and without getting into fresh water as a resource, which will be the next golden commodity and a very golden commodity it will be, of which the US will be a buyer, not a seller).
I'm certain the US still has a lot of history to make but unless it gets excellent leadership soon its power will wane and fade quickly due to the enormous consumption needs of the nation. As with the war for oil in Iraq the U.S. could probably try to take what it needs in the days to come by force but I think that they know that any major aggressive action against most nations will be met with total war by 21st century technology (biological warfare, nuclear warfare, etc) and will end in the total destruction of the US, if not the rest of the human race.
So I guess it depends on what you mean by world history. If you mean it in the shortest tense of the word then the Americans are a player. If you mean it in any other sense they certainly are not, as it was only a pair of centuries ago that they were shaped and formed by the British (who certainly are a major player in world history).