Back in the days when Harry Potter was the only wizarding world franchise, not much was understood about Grindelwald other than he was German and Dumbledore defeated him in 1945. With such scant details to follow, most fans understood the Dumbledore/Grindelwald showdown was an easy shorthand explanation for the magical world's reading of World War II. But with Fantastic Beasts & Where To Find Them revealing the wizarding world was part of World War I, that began to change. Now, with Grindelwald's WWII reference in Fantastic Beasts 2, Grindelwald has a new argument for his cause. Warning: Spoilers for Fantastic Beasts: The Crimes of Grindelwald follow.
Grindelwald's rise is still a parable where he is analogous to Hitler. His ultimate plan, as Vinda Rosier blurts out at one point, is the kill all the muggles for the crime of being born the way they are. It's just, unlike World War II, where most of the anti-semitism feelings were ginned up with false claims and racist rhetoric, Grindelwald actually has an argument he is making which wizards and witches can feel better about what they are buying into. At least, it's an argument that makes them feel good about the idea they should rule over the Muggle populations.
By adding to the narrative the wizarding world was caught up in the Great War like everyone else during the 1910s, the series gives wizards and witches a reason to feel an aversion to getting involved in another muggle-based war. Grindelwald's reveal during his speech then includes the future holds another great and terrible war, with images of the London Blitz and Hiroshima punctuating his rhetoric. Suddenly his argument he's just doing this because Muggles cannot be left to run the planet on their own feels like something most wizards and witches with even the smallest of superiority complexes can easily buy into.
As Grindelwald reminds Rosier, they "don't talk about" the ultimate plan to eradicate Muggles from the earth completely. This is something to worry about down the line. The way to get people to join their cause is to talk about things like "freedom to live openly," which those living in the oppressive American wizarding world, like Queenie, can appreciate.
But even though this complicates Grindelwald's message making it easier for those who would be against him otherwise, it's a bit confusing for audiences to process. So Grindelwald would have stopped the Holocaust? Well yes, but he would have replaced it with a far larger one, eventually killing all non-magical peoples across the world. He really is "Wizarding World Hitler." He's just using Hitler's own war to promote his own.
Grindelwald is seductive to those who are unhappy and are looking for a message about how their lives can be made better. As MACUSA President Seraphina Picquery notes, he is too persuasive by half, which is why they cut out his tongue. Too bad the tongue they cut out wasn't really Grindelwald's. It would have made the rest of this story so much less complicated.