During the Second World War, and before the attack on Pearl Harbour, Britain was in desperate need of American supplies and arms to keep the war effort afloat. Part of the issue here was that Americans did not wish to get involved at all in European affairs as this was still her era of neutrality. In order to continue justifying aid to the war effort Roosevelt had to continue drawing concessions from the British in return for assistance. One of these concessions was the dismantling of the preferential trade network Britain had with her colonies (we all know America loves free trade). Churchill agreed, this was really the moment when the British empire ended.
Yes, i've read about this in some history book. This case was used as example of geopolitical wisdom: young imperialist power (USA) strangled old imperialist power (British Empire) without any war and even staying the "ally" of the victim. So, British Empire began to crumble soon after its formal victory in WW2.