Here is one scenario, to quote a very prominent alternate historian.
"America comes in, sucker punches Hitler in the face, and then nukes Europe while jumping into the ocean".
That is basically what would happen. If Britain keels over to right-winged politics "due to weakness" (quotations for emphasis), then America would become increasingly LEFT-wing, more so if this German-British alliance attacked the US. Left-wing politics would fuse with American jingoism, as people start to compare the left's movement with the American revolution, and the war just becomes perceptively no different from the war of 1812 in terms of propaganda. America would probably (like 90% chance) side with the Soviet Union for the duration of the war, and the two would absolutely clean house with Fascist Europe. Left-winged politics would completely dominate global politics in the modern world. Cold war would probably happen, fear of nuclear annihilation, Communism would be realized as very destructive and violent, but the world would be divided between Far-Left and Centre-Left, rather than Far-Left and Centre-Right.
Worst case scenario, communism takes over the world. Best case scenario, we really nail down egalitarianism and democracy. Likely scenario, the modern world just becomes the living embodiment of Tumblr, take it how you will.