Nazi

Nazi Germany is the common English name for the German state between 1933 and 1945, when Adolf Hitler and his Nazi Party (NSDAP) controlled the country, which they transformed into a dictatorship.

Doom I
In released versions of Doom prior to V1. 4, E1M4: Command Control (Doom) contained a swastika design on the floor of one room. According to John Romero, it was placed there as a homage to Wolfenstein 3D.

Doom II
According to the secret level 'interludes' the Doom Marine visits a patch of hell that has been built by humans...
 * Congratulations, you've found the secret level! Looks like it's been built by humans, rather than Demons. You wonder who the inmates of this corner of Hell will be.

He discovers it is filled with Wolfenstein SS.

A second Secret Level found within the first states:
 * "CONGRATULATIONS, YOU'VE FOUND THE SUPER SECRET LEVEL! YOU'D BETTER BLAZE THROUGH THIS ONE!

Inside these levels are tapestries with swastikas, and paintings of Adolf Hitler.

Doom II GBA
In Doom II GBA, images of swastikas and paintings of Hitler are replaced with the Wolfenstein Symbol (from Return to Castle Wolfenstein), and the Nazi General Wilhelm Strasse.

Wolfenstein RPG
William J. Blazkowicz battles Nazis, who summon the Harbinger of Doom from Hell. He is able to defeat it with the Spear of Destiny, taking out it one of its legs and arms. Before returning to hell it curses his decendents (B.J. Blazkowicz and Stan Blazkowicz).

The Harbinger of Doom would later return as Cyberdemon as seen throughout Doom, Doom II, Doom 3, Doom RPG, and Doom II RPG.

Spear of Destiny
In Spear of Destiny and its expansion packs. The final bosses are demons summoned from Hell.

In Spear of Destiny, William J. Blazkowicz is transported to Hell, wher he fights ghosts and the Angel of Death in a region of Hell called the Death Dimension. He is transported back to Earth just outside Castle Nuremburg (the grass burned around him). Goat-skull images of Baphomet/Icon of Sin, can be found on the walls throughout the level.

In one of the two expansions, B.J. is again transported to Hell (in level known as "Devil's Advocate"), where he fights SS ghosts, and another demon called Devil Incarnate.

In the second expansion, B.J. was transported via magic spell to a Union Aerospace Corporation base on or near Mars (or possibly Tei Tenga) far in the future where he fought the Devil Incarnate for the second time.

Doom (novels)
Flynn Taggart has a tendency to compare things back to Nazi Germany, and even mentions Adolf Hitler. There is a reference to the Swastika room in the Command Center, from early versions of Doom: Knee Deep in the Dead.