As I said, Lend Lease played a significant role in the Red Army's victory over the Wermacht on the Eastern Front, but it was not decisive, imo.
Material aid to the Soviet Union did not begin in earnest until 1942. By that time, the Wermacht had already suffered a signifcant defeat in front of Moscow at the hands of the Red Army.
Despite suffering catastrophic losses in the enormous battles of encirclement at Bialystok-Minsk, Smolensk, Bryansk and Vyazma in the summer of 1941, the Red Army was able to bring the Nazi legions to a halt just outside Moscow. As the Germans lay bleeding and freezing in the snow around the Soviet capital, the Red Army counter-attacked with fresh forces from the Far East, and very nearly succeeded in completely routing the German invaders.
The Wermacht never fully recovered from the wounds it suffered in the Battle for Moscow.
Gerd R. Ueberschär takes up the narrative:
By the end of December, the Red Army had finally eliminated the threat posed to Moscow by the German divisions. The Wehrmacht had been repelled up to 280 km. (175 mi.) from Moscow and had suffered heavy casualties. BARBAROSSA had failed. At the same time, the myth of invincibility of the Wehrmacht, which had grown accustomed to victory, was shattered. By 31 January 1942, it had lost almost 918,000 men, wounded, captured, missing, and dead—28.7% of the 3.2 million soldiers involved. The Wehrmacht never recovered from these heavy personnel and materiel losses. The Red Army, too, suffered heavy casualties. By the end of 1941, 3.35 million Soviet soldiers had been taken prisoner by the Germans, but the USSR had not collapsed militarily. The Red Army's victory before Moscow also considerably enhanced the politico-military importance of the USSR. The successful warding off of the German attack—before Lend-Lease had started on a large scale—strengthened the morale of the Red Army and the Soviet population. It also enhanced the international reputation of the USSR, which on 1 January 1942 was a major co-signatory of the United Nations Declaration.
About the German Invasion of the Soviet Union
Another perspective:
Why did Barbarossa fail? Glantz believes that "the most significant factor in the Red Army's ability to defeat Operation Barbarossa was its ability to raise and field strategic reserves." The German panzer groups annihilated one Soviet army after another, only to face a new wall of defenders. While the defending Soviet forces around Moscow received about 75 division equivalents during October-November 1941, the attacking Germans received not one new division. Stalin, unlike Hitler in 1941, believed in total mobilization. Furthermore, the bad weather explanation has been used by Germans to conceal a host of operational errors and reckless contempt for a resilient enemy.
Amazon book revue, Barbarossa 1941 by David Glantz
Bear in mind that Russia (and other nations of the former USSR) is a vast country with impressive mineral wealth, abundant natural resources, and vast fertile regions far beyond the reach of either the Wermacht or the Luftwaffe. Additionally, Nazi characterization of the Russians as Untermensch awakened Russian patriotism and fueled a burning hatred for the Nazi invaders. Finally, the Wermacht had met not only its match but its master in the Red Army, which had quantitative and qualitative superiority in its weapons, and far more available manpower.
To summarize, even after suffering almost unbelievable losses in the initial Nazi onslaught, the Red Army did not collapse, but was able to stop the Wermacht and inflict a serious defeat on it long before receiving any appreciable aid from either Great Britian or the United States.
When it invaded Soviet Russia, Nazi Germany was doomed.
S