The major reasons why the US was able to assert itself after World War II were that 1) Domestic war spending in the US had greatly stimulated the economy, 2) the US had been part of the Allies who won the war, and 3) there was a giant power vacuum left by the relative fall of the British empire that was easily filled. 4) The US Military was at its peak and had tons of resources it was able to use to build foreign bases and, they had nuclear weapons :)