Most of the German military was defeated in the east. 54% of the German air force along with 80% of the German army was defeated in the east. The West didn't do much until 11 months before the end of the war when the allies were already winning. American supplies and Western Allied bombings on Germany were important but the Soviet Union destroying the majority of the German military was the main cause of Germany's defeat in WWII.
Eastern front saw by far the most bloodiest and deadliest battles in history of warfare with tens of millions of people killed.
The Siege of Leningrad, The Battle of Stalingrad and The Battle of Kursk were the 3 single most pivotal battles of the Eastern front and WWII in general. The western front did not experience anything like in magnitude or significance. That is not to say that the western allies efforts were not contributing, they certainly were aiding the anti-nazi campaign.
The USSR defeated the nazi war machine by destroying 90% of the Hitler's army with 80% destroyed on USSR territory, alas at a great cost of over 25 million of their own. The statistics are undeniable, with western allies involvement being limited to the shores of normandy, pearl harbour, western front air strikes, aid & equipment to USSR & other allies and retaliatory offensive on Japan (including nuclear).
Furthermore, USSR liberated Mongolia and China from the Japanese before the nukes were dropped and Japan capitulated.
For those who claim that the weapons and equipment supply from USA played a major role - this english proverb should prove helpful- "You can lead a horse to water, but you can't make it drink." It was still the bravery and honour with support of highly industrialised USSR machinery (towards the end of the war) that made all the difference.
Germany already lost war by the time allies landed in France. Most of German losses and all major battles that broke Nazis back happened in the former USSR. No denying it. It is wonder I have to write this here. Many in the west are plainly brainwashed but in 1945 there was no doubt as to who defeated Germans and it was Red army.
The Soviet front was the largest of the war and the most important theatre. It didn't matter what the Allies did in Italy or France or the North Sea. If the Germans lost in the east, they would lose the war, period. However, if the Germans won in the east or did not fight in the east it would be impossible for the western allies to win. The vast majority of war output was directed to the Soviet Union. The Soviets absorbed all of it, and managed to turn the tide on the Germans. If it weren't for the landings in Italy and France, It's possible that all of Europe, save Spain and England would have fallen to the Soviets. The same cannot be said for the west. Western supplies were important to the Soviets, but the Soviets had an insanely huge domestic war effort too, which dwarfed even vast amounts of aid.
As I haven't little time I will make this short with little fact, having studied world war II in much depth, I can state (without doubt) that the Western Airforces had a much larger impact than the Soviet Union did. The initial failures being ignored, it was the strategic bombing of the Nazi synthetic oil production and transportation network that severely lamed both the Luftwaffe and Wermacht, and provided the Western infantry to push further into 'Fortress Europe." Besides, the only reason the Soviet Union was able to hold such a successful 'counter' invasion into Nazi Germany was as a result of the western air aid provided. This is a quick (not necessarily entirely correct) list of the aid provided:
Machine guns 131,633
Explosives 345,735 tons
Building equipment valued $10,910,000
Railroad freight cars 11,155
Cargo ships 90
Submarine hunters 105
Torpedo boats 197
Ship engines 7,784
Food supplies 4,478,000 tons
Machines and equipment $1,078,965,000
Non-ferrous metals 802,000 tons
Petroleum products 2,670,000 tons
Chemicals 842,000 tons
Cotton 106,893,000 tons
Leather 49,860 tons
Army boots 15,417,001 pairs
It is debated whether the Soviet Union would have survived the German assault without Western Aid. I say they could have, but would most certainly not have been capable of fighting a counter invasion. Also take into account USA's distraction by the Japanese.
We could argue about the importance of any individual battle, but the overarching situation is simpler. The western front and Germany's attentions there utilized the vast majority of their resources. The tremendous investment that Germany put into taking France, assaulting England, and holding these territories against the U.S. is tremendous.
If all of that support had been placed on the eastern front (not to mention all of those supplies), the USSR would have been placed on the heels for quite a while. The reality is that the USSR faced Germany for only a short time, and was on the verge of a several major defeats even with the limited commitment.
The Germans were running out of fuel, food and other equipments, the distances were just too big and not to mention the cold winter, the same problems like during the Napoleon invasion. The Soviets had not the technology to defeat Germany, they got weapons from USA. That's what my grandfather is saying.