During WW1, the british used their foreign currency reserves to loan their allies money to fight the war. After ww1 all sides were financially exhausted and the British never required their allies to repay, on the grounds that they did not have the money, even though they suffered the worst financial losses of all the allies. After ww2, the US was alone among the allies in not being broke, in fact they emerged financially stronger. They required repayment from the British almost immediately. Loans were which finally paid off in 2007. It took 62 years to repay. The requirement to repay the americans was part of the financial problems which meant it was years before rationing was ended in Britain, well into the 1950s.
The bankrupt nature of Britain and the US requirement for repayment mean the British were not just a threat to the US in post war years, but were forced to withdraw from their empire(not necessarily a bad thing), but effectively on terms dictated to by the US. For example when the British couldn't afford to assist Greece, the US stepped as part of the Truman doctrine.

What I'm saying is that the US used the war to destroy not just their enemies, but weaken their allies and make themselves dominant in the post war period. They extended their dominance from the Americas to the whole world.