Hitler's declaration and America's path to Europe

Would it have made a difference if Hitler had not declared war on the United States?

It could have complicated Roosevelt's position, but the assessment is that Anglo-American cooperation against Japan would probably have drawn the United States into the European war anyway.

Hitler's declaration of war is judged to have eased Roosevelt's immediate political task. Without it, American and British forces would still have been fighting together against Japan. In this counterfactual, that cooperation would have made sustained U.S. neutrality toward Germany increasingly difficult. A wider war remains the predicted outcome, although its timing and political path are uncertain.