The infusion of the US brought a lot of new power to the Allies. Whereas before the industrial aid was limited due to treaties and laws, with the declaration of war that didn't matter. Additionally, the US forces swung power markedly. Instead of Great Britain barely hanging on, she swiftly became an aircraft carrier and launching pad for the attack of German held territories en mass and forced the Germans to split their forces more so than previously.

It took time to build up the forces and experience, but the entry into the war turned Europe from a stalemate in the west and main fight to the east into a more even squeeze with the Germans having to fight both sides as well as North Africa, later Italy, etc.