Nazism was first invented by Satan when he was bored. Though he is not a Nazi himself, he is the founder of Nazis and they seek him out for advice on how to kill. He taught this Nazism to Bowser, who in return promised to spread Nazism around the UnWorld. However, Bowser's plan to take over the UnWorld has not been put into effect yet, so Nazism is limited.
Despite this, Bowser has had relative success in spreading Nazism. Bowser has conquered many places, and he has brainwashed the leaders of many more. These places are collectively known as the Bowser Empire, or sometimes the Nazi Empire. Besides the Bowser Empire, the strongest Nazi presence today would be the Grammar Nazis.
In 3178, the Nazis started World War III. As WWIII kickstarted the End of Days, it can be argued that the Nazis ultimately ended all of reality. Of course, they did this out of stupidity, not on purpose. But it turns out the End of Days was not the end, as Quantum Person accidentally the Neotubby Empire.