WW II was over by June '44. Germany had been broken since at least the summer before with its defeat at the Battle of Kursk. It lacked the manpower to continue any sustained operations against what was now essentially the rest of the world. Stalin was already rolling west by this point and D-Day was necessary to ensure the US some of the post-war spoils. Had the US not landed in France, Russian tanks would have been pulling into Paris a year later. It makes Americans feel good to talk about D-Day but the Russians won WW II. The last time the US was involved in total war was the Civil War and even then it was only the south that was really impacted. Parts of the north were hardly effected.
The largest theater by far was the eastern front for sure. And it can be argued that the Russians won the war. But they didn't do it alone. with out America's involvement early on in the form of material help to both Britain and Russia along with keeping Japan from wanting to possibly expand its war, allowing Russia to move many divisions to the eastern front and also keeping 30 or so Germans divisions on the eastern front played a crucial role.
What would have happened if Germany's momentum in 41-42 goes unchecked? Thankfully, Hitler in some ways aided the allies with his dumb moves during the war also. Such a indecision what to focus on in Russia, ( Moscow, then Stalingrad, then Moscow, etc.), impractical focus large expensive tanks, Dunkirk, etc
