You have asked literally one of the most complicated questions in the history of the 20th century. There is no real way to answer that completely in 10,000 characters (possibly not even 10,000 pages).

There is a short answer for when: immediately following World War II.

The simplest answer for why is that the previous Great Powers (UK, France, and to a lesser extent Germany, Russia, and Japan) were financially and physically destroyed by thirty years of World Wars, Great Depression and Revolution. The US came out of it not only almost completely unscathed but at its highest point in industrial output and military strength ever (some argue that it has never been to that point since).

There are many, many, many expansions on this basic idea, that avoiding physical and economic destruction during WWII left the US the most powerful nation. For example, there are some who argue that American ideology or worldview was particularly adept at dealing with the realities of the post-War era, others say that the US had been working self-consciously toward "world domination" (hegemony, empire, etc. the terms vary by school of thought) since the Founding Fathers.

edit for grammar/spelling