From what I know it was Hitler that declared war on the US after they declared war on Japan and that they first really joined when the war was about to turn at Stalingrad. From what I've seen on sites the amount of Americans that fought in the 2nd world war were 16.1 million making up less than 1% of the people that fought in the war. I know a lot of Americans seem to brag about having saved Europe, but did they really at all? Late to action and taking all glory is all I can see
Major supplier of Allied Forces, including both communist and Kuomintang forces, from 1939 onwards. The Pacific theatre and North African Campaign are overlooked theaters of war because of the lack of cultural significance to many westerners but some of the fighting done in the Pacific by Americans was vital to the ending of conflict with Japan such as Midway and Battle of Coral Sea. There really was no other naval superpower capable of taking down Japan at the time. Germany's u-boats were not meant to county true imperialistic naval superpowers like USA or Japan but stifle the flow of men and supplies.
No one disputes that the bulk of the land war was fought by Chinese and Russians but such is Asian warfare during a world war. But while volume of fighters and casualties may have been outcome determinative in Asia, the European theatre greatly benefited from the press of UK, France, and USA to ensure the cold war era had a balance of powers. Otherwise, a super hegemony of Russia and China would have surely paved the way for ultimate socialization and economic ruin for centuries to come. America's standpoint as the political victor of WWII is exaggerated but one that mankind can thank them for seeing as the fall of communism was imminent. The political and social upheaval that would have come out of a fully USSR controlled Europe can't be even fathomed although it would be like imagining everything East of Spain being equivalent to the livelihood of an eastern european country. That is hindsight though. Socialism is a great political ideology. I've lived in Cuba for a bit and absolutely love the lifestyle and camaraderie of genuine communists. But sadly, the post WWII cadre of communists out of USSR and China did nothing but atrocities to their own people and one could say for lack of genuine embracement of true communist principles--mainly for fear of losing their own personal power and being a dominant hegemony. I'm not saying the cold war was good but the exaggerated political victory by the Americans is not so exaggerated if you see WWII as a firm starting point for the rest of the 20th century. Although many atrocities were committed in the name of Mcarthyism or anti-communism by the USA, the ending point of the USA in WWII can be said to greatly increase the livelihood of westerners for the most part. Westerners also create most of the media and drive the focus of academic discussion sometimes and therefore the US, UK, Canadian, Australian, New Zealand, and French involvement in WWII is paramount in media and academia for its interest to economically affluent audiences. Although you're perception of this exaggeration itself shows your susceptibility to it as a consumer of western media. Many great works have been written in Chinese and Russian on their achievements of the land battle in Asia but unless those cultures dominate the minds of westerns, they will not dominate the media.
EDIT: check out these awesome Russian aviator women.
The US was crucial to the development and ending of WWII. Nobody really disputes that. But it was a world war in which the (positive and negative) contributions of everybody were crucial. It certainly wasn't the only crucial contribution- and it shouldn't be regarded as such.
If you're interested in who is perceived at winning, this is interesting (though I'd be interested in Russian perceptions too).
https://yougov.co.uk/news/2015/05/01/Britain-America-disagree-who-did-more-beat-nazis/
Personally, places of near crucial importance- The Soviet Union suffered horrific casualties and were of huge importance. Britain was also crucial. By staving off invasion, Britain remained a place from which an offensive could be launched and a beacon of hope for Europe. The US did arrive late (I've heard it said that the USA ended the war wealthier than at the start, whereas Europe was decimated. This kind of suggests that they could have done more). Their economic and military contributions were vital. The atom bomb (though a subject of debate), I don't think was vital.
I reckon that there's a perception that the US 'won' the war, or was the deciding factor, in part because the Cold War meant that celebrating the achievements of the USSR was politically incorrect, and partly to legitimise in retrospect the horrific bombing of Hiroshima and Nagasaki. To attempt to value each country's contribution however isn't particularly useful. Everywhere was important.