Basically the Americans went back to their pre-war policy of fucking over and undermining the British whenever possible. To make sure the British Empire dissolved and America was to secure its place as the new super power.
They were very harsh on Britiain and France for post war depts, meaning Britian had to spend what little money it had paying off the Americans instead of feeding people, and the Americans also withdrew thir direct food support that hey had sent during the war that made things even worse.
All of this while they were investing heavily into building Germany back up as a buffer to the Soviets.