My answer was to who "loosened up Hitler" for Americans. If we measure the contribution (in lives and outcome) it was the Soviets who contributed the most to Hitler's defeat, not GB.

Regarding the cooperation:

IBM (an American company) facilitated the holocaust by technology for concentration camps. Heinrich Himmler described Henry Ford (an American industrialist) as "one of our most valuable, important, and witty fighters". There was a strong pro-Hitler sentiment in America these days in influential circles (we can find American sources on that topic). USA and GB entered the war when their interest was in danger, which is to be expected from foreign politics. It was business after all (from their perspective).

But we are digressing from the subject of who "loosened up Hitler". If we compare thousands of GB soldiers to 27M Soviets who died in WW2, we can only say that the Soviets "loosened up Hitler".