Fighting on the eastern front was the heaviest of the entire war. While all the other fronts were joint forces of the British, Americans, French, their hosts of colonial subjects, and minor allies all joined together, all eastern front fighting was done by soviets without direct help from other countries.
Even in the pacific front, A huge portion of Japanese military power had to stay in China, Manchuria and Korea because of the possibility of Soviet invasion from the north. If it weren't for the USSR, the American/ANZAC forces would have had a lot harder time fighting in the pacific.
The Nazis lost more airplanes in a few battles on the eastern front than they did during the entire Battle for Britain. WW2 was won only because of immense Soviet sacrifice. The west didn't stand a chance without the USSR.
The USSR was responsible for the defeat of some 80% of the German army. Most of the German forces were engaged in the east with a minority fighting in Africa, Italy, and France. Western Allied bombings helped, but it was in the east where Germany lost most of its army. In June, there were around 3.5 million German troops preparing to invade the Soviet Union. At the same time, there were only around 150,000 Germans fighting in Africa.
The eastern front split the German forces on to two fronts and so slowed German advances into France and so allowing for serious preparations to make way in Britain, as the British expeditionary Force was incapable of waging war on the western front at the time. If it had not been for the USSR's involvement the entirety of France would have fallen and the Battle of Britain would have been lost giving Nazi Germany a greater foot hold on the Atlantic.
In addition to this the USSR was fighting from the beginning of WWII and were the ones who had ultimately captured the Reichstag, the western allies would have been defeated without the USSR.
World War Two was a war of resources. The Soviets destroyed the vast majority of Axis resources. The Soviets are the primary winners of the war. If Germany and the Soviets had no intentions of going to war with each other, the Allies would have lost, hands down. On the other hand, even if D-day never occured and second front was never opened, the Soviets would have still have pushed to Berlin, and beyond. In fact, if a second had not been opened, perhaps all of continental Europe would have been brought into the Warsaw Pact after the way. The same could not be said for the allies in the opposite scenario. People who say that America was important need to learn more about real history, I.E. Not the kind that is found in American public school textbooks.
Historically speaking the Reds were the only reason the West was able to win the war. Firstly, by the time D-day happened the Soviets were already on the advance towards Berlin, if anything the western invasion just helped keep the Germans tied up. Even if they had only pushed the Germans to the Soviet border, the reason the Germans invaded was to try and get the oil fields, and so by denying them that the USSR played a key role. But in fact, if you just look at a map the Red Army alone covered more ground in Europe and Germany itself than the Western Armies combined.
Although USSR did play a significant role in limiting movement of the Germany military, it was the Western allies that defeated Germany. As you all may know, Germany had a non-aggression pact with the Soviets before the war, ensuring that the Germans would only have to fight the French and the British, but then Hitler got too greedy and attacked Russia, which was the worst mistake he ever made. BUT, this does not mean the Soviets caused the Germans' defeat.
Now the Nazi Germans were forced to fight the war on two sides, which diverted crucial German military troops that may have caused the fall of U.K., to the Eastern side to fight against the Russians. After D-Day, with the Americans leading the Allied effort to end the war, there was no way the Germans could way. After all, modern trench warfare lead to a new way of fighting, "a war of attrition". During a war of attrition, the side with the greater economic power usually wins unless there's foreign intervention. Now in this case, the Soviets were helping the Allies after the Nazis double-crossed them.
Although Stalin may have had more energy during the Yalta Conference compared to Rooselvelt, and helped capture a large contingent of German troops at Stalingrad, the Allies kept marching east toward Berlin. The Allied force had greater economic power than USSR (which was not at its peak yet), had more military strength in terms of troops, logistics and technology, and had the best thinkers in the world that included scientists, engineers, chemists, politicians, strategists, and diplomats.
There is absolutely no possible way that USSR could have triumphed against the Germans without Allied support. On the other hand, the Allied force may not have either without Soviet support, but because USSR lacked the sophisticated technology, had economic problems (including rampant inflation), and motivated many of its troops through fear and intimidation, USSR would have fared worse on their own than the Allied forces.
We won the war, and this allowed U.S. influence to reach into the very corners of Europe, that was partially held down by the Soviets. The lasting influence shows how the Allies won the war and not the Soviets (although may Soviet leaders claimed so).
For those of you who say that the USSR did more than the British and Americans you are completely ignorant. I mean no ill-will towards Russians, but let us be honest. When Hitler and Stalin made the non-aggression pact one of the agreements was for both countries to conquer Poland and split it between each other. That is what happened when Poland was conquered by the Soviets and Nazis. When Hitler invaded the USSR the Soviets were devastated in a matter of months. Moscow was taken by winter of 1941. Stalin was begging on hands and knees to Churchill and Roosevelt to send weapons and supplies and invade Western Europe as soon as possible to take the pressure off Stalin. Even with all the supplies the Soviets struggled constantly to hold Stalingrad being pushed to the Volga several times. Manpower and winter attrition alone saved the USSR from destruction. When the Western Allies invaded Normandy Hitler sent most of his armies to the west including troops from the Eastern campaign to hold off Patton and Montgomery. Despite this the Soviets took very heavy losses trying to take Berlin. Eisenhower was ordered to let the Soviets take Berlin even though we could of easily beat them there and taken it sooner. The Western Allies controlled 75% of Germany by the war's end and the Soviets had 25%. Not only did the Soviets contribute to the start of the war they also took thousands of losses for every single foot of ground, but only achieved victory because they had so much manpower to spare.