Grindelwald is meant to be a Nazi allegory. He spends all his time talking about the "superior race" of wizards, and believes that all Muggles must live in servitude, or just be wiped out. Grindelwald was even an Austrian who later moved to Germany and set up his base there.
In The Crimes of Grindelwald he shows visions of massive armies of tanks, the Blitz on London, the bombing of Hiroshima, etc. All good evidence that humanity is evil... except for the fact that none of it happened yet. The movie takes place in the 1920s, decades before any of those things were even dreamed of. Either he could see the future with incredible accuracy (something even the most talented seers couldn't), or he knew what would happen because he was planning for it.
There's some pretty clear details connecting Grindelwald to WWII Germany. He was defeated by Dumbledore in 1945, the same year the war ended. He was then imprisoned in Nuremgard (similar to Nuremberg). Additionally, who was he fighting? Where are the biggest wizarding schools located? England, the US, France, and Russia - aka, where Germany went to war against. MACUSA was even sympathetic to his anti-muggle ideas, which may have been why the US took so long to get involved.
Grindelwald lacked the resources and numbers he needed for his plan. However, by influencing a muggle government and helping the Nazis rise to power, he could start a brutal, bloody war that would exhaust both wizard and muggle governments, leaving them ripe for takeover.
There's also the fact that wizards have no protection against muggle weapons. The bombing of civilian areas was relatively new in WWII, and wizards would be even more unprepared than muggles. Attacks like the Blitz could wipe out hundreds of wizards. In the Harry Potter books, almost every pureblood/halfblood wizard we see doesn't live in a city. [Meaning, those "types"] Wizarding families were the ones who would survive.