"The Killing Game"
Star Trek: Voyager
This two-part episode features the aliens known as the Hirogen, hunters who hounded Janeway and her crew for a while (in the ship's defense, the Hirogen had a sweet data plan complete with tech specialist Andy Dick). For one reason or another, the Hirogen choose to stick the crew in a holodeck program where they act out WWII, and everyone pretends this is completely logical, even when the lead Hirogen intones, "This is not a game!" even though, ah, it clearly is.
And incredibly, this is not the only Star Trek two-parter to features aliens masquerading as Nazis! (There was also Enterprise's "Storm Front.")
Okay, and so while I'm being somewhat flippant today, "Killing Game" is actually a lot of fun (which is always odd to say in relation to Nazis; see also: Inglourious Basterds), and is also indicative of how so much of the franchise after the original series, which was clearly modeled on the Cold War era, was kind of fixated on WWII (Deep Space Nine's Dominion War, certainly).
Also, in the program, Janeway becomes a leader of the French resistance, while Chakotay is an American soldier. This is ironic, because in some ways the Maquis (as the name implies even in an obvious way) were modeled after the French resistance and Starfleet would be analogous to Americans, meaning for "Killing Game" Janeway and Chakotay swapped backstories.
The Hirogen, no matter if I've spent a little too much time making light of them, were actually some of the best aliens Voyager featured. One of them even gave the famed Species 8472 (the CGI beings who at one point were kind of set up to be the show's answer to the Borg, until it relented and just mad the Borg the show's Borg) a run for their money. A lot of fans tended to question when the series featured any one adversary for an extended period of time, but it made a great deal of sense, especially if the enemy in question was nomadic or sprawling in some other way, as the Hirogen were and certainly the Kazon (the guys who looked like even grubbier Klingons and were most famous for involving themselves in the treacherous affairs of Seska, who helped make the early seasons some excellent material to revisit) as well.