History

I mean, the Americans weren't exactly sworn enemies of the nazis. Their "alliance", for lack of a better term, just wasn't this blatant. It manifested more in running significant cover for nazis post-war; even shipping them to high places within the US government and military complex; undermining de-nazification efforts, in anti-communist ops such as GLADIO, along with the red scare and its purges of Americans with communist leanings from their jobs. Also in commanding that the factories belonging to american companies on german soil not be bombed, paying out tens of millions of dollars to those companies when their factories were destroyed, even though the sole purpose of those factories was to manufacture and sell weapons to the "enemy". There were straight up nazi rallies in America, swastika flags and all. If America was hostile to fascist ideology or leadership in any way, such rallies and corporate collaboration quite simply would not have been tolerated.

America didn't enter the war to defeat Hitler, as if there was some switch that flipped when the axis powers "went too far". This just doesn't make sense after everything the Nazis had already done, all the gains they had made that would have threatened the US if the US had truly considered them enemies. No, the hope was always for the Nazis to crush the worker's movement in Russia. The US only entered once it became clear that the Nazis were out-matched and the best possible outcome for the US would be to prevent the USSR from being the sole benefactor of Hitler's defeat.