The Soviets managed to turn back the Germans at Stalingrad and again at Kursk, but don't act like they didnt lose anything while doing so. For a good amount of time the Eastern Front was the only actively grinding front, both sides taking massive casualties. Stalin knew he was losing wayyy to many troops for the massive and under equipped Soviet army, and heavily pressured the Allies to open up a different front to help relieve pressure on his army. They did that with the Italian front and later the Western front. America's arms and troops were absolutely instrumental to both of those, and assisted the allies phenomenally. The Allies never could have taken Normandy without the US, and Italy would have been a lost cause without the US either. The other allied countries didnt have the manpower or industry to wage a successful operation against Hitler and Mussolini's war machine.