Aye, it is true that there is a very stong tendency in Hollywood movies in particular to depict the US as the single handed winner of WW2 without whom the whole world would have fallen into the abyss of fashist terror.
The truth (admittedly a truth that is easier to diagnose with the benefit of hindsight than it probably was at the time the war was going on) is that there wasn't any realistic chance for nazi Germany to defeat the Soviet Union or rather to conquer it, murder most of the population and thereby gain "Lebensraum im Osten" (space to live in the east) the way Hitler pictured in his disgusting schemes.
Many Germans underestimated the Soviet Union and its military capacities for several reasons.
First there was WW1. The revolution in Russia caused a Russian surrender at Brest Litovsk to terms Russian politicians probably would have never agreed to, but for the fact that on the one hand the German defeat was foreseeable so the terms were likely to be very temporarry and secondly because Lenin and others thought more in class rather than nationalist terms therefore attributing less priority to national interests.
Secondly there was extremely expensive Pyrrhic victory of the Soviet Union against the much smaller smaller Finnland in the winter war of 1939-1940 which contributed to the immage of utter incompetence of the Soviet military.
Last but not least the crushing Soviet defeats of Soviet troops in the first months after the German attack reinforced that image (though a good deal of these defeats is to be blamed specifically to Stalin's making the same mistake as Hitler did by issuing "don't give an inch" orders that allowed for large troop contingents to be surrounded and captured rather than draining the attackers with a prolonged fighting retreat.
Unlike in case of 1917 there was no significant opposition in Russia to launch any kind of revolution (Stalin's brutal murdering of anyone even vaguely suspected of dissenting in the 1930s had made sure of that), and moreover the genocidal war of annihilation committed by the German troops also worked to create a "do or die" kind of mindset against the invaders.
The war of annihilation launched by Germany pretty much left the Soviet population nothing to gain at all from a ceasefire or peace with Germany. Therefore even in the worst case of a capture of Moscow in late 1941 this would not necessarily have resulted in a surrender of the Soviet Union. Production facilities had been evacuated beyond the Ural (well out of reach of the German Luftwaffe that lacked any long distance bomber) and of course the weather conditions were working against the invaders too. Napoleon's capture of Moscow in 1812 was considered even back then as an example that the capturing of the capital was not such a matter of course cause for a surrender as would have been the case with most nations in Western Europe.
The Soviet Union's military and industrial capacity surpassed that of Germany by far and the quality of the arms too was (contrary to ocassional claims very good). The T34 for example was probably the best middle tank of the war, the Iljuschin Il-2 Stormovik one of the best planes for ground support (and the most frequently produced plane in history) and the Katyusha rocket launcher, in spited of limitted accuracy proved highly effective to prepare infantry and tank attacks and demoralize the opponents.
The American contribution to WW2 was important (this refers also to the lent and lease goods provided prior to the Japanese attack on Pearl Harbor), very likely shortned the war by years and certainly ensured that large parts of Western Europe were not under the controll of the Soviet Union at the end of the war.
With regard to the outcome of winning and loosing however, the American contribution was not the decicive factor. The Soviet Union would have won even without the American entry into the war. Post WW2 Europe would have been entirely different however.
With regard to the losses the Eastern Front of the European theatre of WW2 was on an entirely different level compared to the western front. The losses of all western nations combined make for less than a tenth of the losses suffered by the Soviet Union (roughly 16 times the combined losses of both sides of the American civil war) and that is (get this!) only true if one counts the military death only ignoring millions and millions of civilians murdered in a genocidal war (and German losses on the Fastern Front were more than six times as high in the East than in the west and exceeded the total losses for the entire war in the west in every single year from 1941 to 1945 in the East.
Here is a graph to illustrate the difference (again, that one does not cover the millions of civilians brutally murdered who in terms of numbers again exceed by far the military deaths of both sides in the west):
Long story short, America's entry in WW2 did not determine its outcome, but It was absolutely crucial for the political landscape of post WW2 Europe an the world.
