2 weeks into the war against the Soviet Union, Field Marshall Manstein realized that the Germans could not win the war. He came to this conclusion so quickly because of 2 observations: 1) The typical Russian soldier was fighting with much more tenacity than the Germans had imagined to be possible. 2) German arms, and tactics were incapable of penetrating the defenses around Leningrad. (They simply were not prepared to sacrifice the amount of men and material that would be consumed in the effort.)
It remains baffling and incomprehensible why the German General Staff did not plan for a winter war. Even if the Soviets were to surrender within the 5 months that was expected, millions of German soldiers would need winter clothing.
However, there was absolutely no way that the Soviet Union could have defeated Nazi Germany without the American commitment to defeat Germany, if necessary, alone without the Russians.
Lend Lease was not a decisive factor in the outcome of the war. Each major Soviet victory on the Eastern Front -from the Battle for Moscow in December 1941 to the destruction of Army Group Center in 1944- was preceded by a major development by the Americans.
1941 — The ultimatum put upon the Japanese by the Ameicans in the autumn of 1941 was certain to lead to a war between Japan and America. No sooner had the Japanese fleet left its ports to attack Pearl Harbor, Stalin ordered the transfer of 40 divisions from the eastern Siberia to the Battle before Moscow.
1942 — American invasion of North Africa in early November triggered the diversion of almost 200,000 German troops from Eastern Europe to North Africa, along with hundreds of tanks. Several divisions were deployed to complete the occupation of France, In Egypt, the Battle of El Alamein was fought and won by the British. All this occurred before the Soviet counterattack at Stalingrad began.
1943 — Allied success in Sicily led to the surrender of Italy in July, while the Battle of Kursk was going on. In response, 12 German mechanized divisions were diverted from the Eastern Front to occupy Italy and prepare for the Allied invasion of Italy.
1944 — Normandy forced the Germans to deploy over a quarter of a million men in an effort to contain the invasion within the Normandy peninsula more than two weeks before Operation Bagration began.
The Allied bombing campaign did considerable damage to the German industrial base throughout the war. By January 1945, there was so little left of it that the allied bombers concentrated on bombing civilian targets for the rest of the war.
Without the Allies (particularly the Americans) the Eastern front would have continued indefinitely with the line of control between the warring parties running roughly from Leningrad to the Caspian Sea. Germany could not conquer all of European Russia; but it didn’t need to. A significant number of Ukranians, plus almost all of the Crimeans and the peoples of the Baltic states preferred Nazi Germans to the Bolsheviks and Stalin’s nationalists.