America Won World War II is a form of Hollywood History in which a story implies or outright states that the United States single-handedly won World War II. Sometimes, it's unintentional; the viewpoint or focus is simply too narrow for the audience to be reminded of the bigger picture. Other times, though, it's a blatant example of Hollywood History. This can be justified in that US history classes are prone to teach a US-centric version of history and it's super patriotic.
When this trope is in play the efforts and contributions of the other Allies are downplayed. Egregiously, the Soviet-German war is considered a sideshow — if it's even mentioned at all. This is compared with a majority of the German forces being focused on the Eastern Front and up to 75% of all German casualties coming from these offenses.
If any other Allied nation-state are suggested to have actually done anything to fight Germany it will be the UK, which will be portrayed as (after the Poles and French got their asses handed to them) having kept the hopeless fight alive until the USA joined in and saved the day. The non-European Allies have it even worse. When was the last time you saw an American film about the Sino-Japanese War, or Filipino guerrillas? All these oversights are at least partly a result of the Cold War making American educators and filmmakers cautious to glorify the Soviet Union or China (especially Maoist China), since this kind of stuff was virtually illegal in the United States lest one be put on the Hollywood blacklist.
In particularly nasty cases, films based on actual WWII events will be warped to make the most prominent characters into US soldiers — see Steve McQueen in The Great Escape and, perhaps most infamously, the film U571. It will occasionally even be said that WWII only began in 1941, when the United States entered the war. Other factually wrong but more playful revisionisms are for example the Americans killing Hitler in Inglourious Basterds.
The D-Day landings are another good example. Many American-made productions will focus solely on Omaha Beach. The carnage that ensued is a favourite among producers, since it emphasizes the sacrifice Americans made during the war — but doing so gives the impression that Omaha Beach was the decisive turning point that led to the Allied victory in Europe. The focus on Omaha Beach is also partially because Saving Private Ryan did it, other games/movies/TV shows want to replicate its success.
Cases of this trope are not limited to the European Theater. Most films featuring the Pacific theater only focus on the naval and air battles fought by the U.S., appearing as though they were the sole force fighting in the Pacific. In reality Chinese, Soviet, Indian, British, and Australian forces played significant roles against overwhelming forces in atrocious conditions, and many other countries contributed as well.
It may not be a strictly American trope. The British may exaggerate their role in the war as well, with an additional jab that the Americans were not only late to the party but also stole all the credit, and additionally only joined in when they knew who was going to win (technically true as the Americans knew the allies would win if America joined them in the war). Russia also gets this to an extent (despite doing most of the fighting); there, you'll find claims that WWII lasted from June 22, 1941 to May 9, 1945 — when this was actually just the duration of the war between the USSR and Germany. Which, ironically, sells the Soviet Union short, because their successful campaigns against Japan in 1939 and late 1945 aren't included in that time frame. But, frankly, American movies and shows dominate (or at least believe they do) the global entertainment market, and of course American filmmakers usually prefer to make movies about American soldiers and battles.
Though there is something to be said for, and these being elements that most of media and real life overlook, the mass production of weaponry and armaments in the US, and the American contribution to peace deals after the war.