What's the source you read it in? also, what's the german phrase you're referring to? it would help if you could distinguish between wir sollten den Sumpf drainieren and some version using trocken legen.

Could you give me any examples? I have the complete text of Hitler's speeches in German and the phrase 'drain the swamp' never appears. The closest you get is 'draining swampland' and 'drain our swampland', both instances in the same paragraph of the same speech. And he is literally talking about draining swamps and land reclamation:

“Then we intend to raise each individual’s standard of living. We want to drain our swamps, reclaim and improve unproductive land, if possible put our Volk in a position to provide for its own needs, enable the peasant to reap the maximum from his soil, put the manufacturer and industrial worker in a position to work as productively as possible, supply our country as far as possible with man-made substitutes for the raw materials it lacks. By building roads, digging canals, draining swampland, and installing dams and sluices, we are accomplishing constructive work which has a right to claim our energies.

-Adolf Hitler, April 4, 1934.

This was not an idle concern. Germany had experienced famine in the great war less than twenty years ago. Some people had also had trouble feeding themselves in the Great Depression. Hitler had a highly ecological view of the world and securing food supply was something of an obsession for him. He also might be borrowing some rhetoric from Mussolini who spoke about similar things in his Battle for Wheat. But even though some of the swamps were near Rome, Mussolini never made the connection between swamp and national politics as normal. He did use it more metaphorically but he usually meant 'make something problematic or unproductive useful'.

The only time I can think where this even remotely works is when Hitler describes the Russians as a sort of swampman. But this is again related to his idea of turning land into agriculturally productive areas. The very next sentence he goes on to talk about how they're making farms despite the resistance. He is talking about the Russians as a sort of natural problem, like weeds. While this absolutely reduces them to subhuman and in need of extermination, it's hard to draw a direct line to any modern rhetoric.

The phrase 'drain the swamp' as a political call to reform was uniquely American in the 1930s. It was uniquely resonant with Americans because DC was literally built on a swamp. (It also has a frequent rat problem.) The earliest use does come from the interwar period but it was a social phrase. The earliest attested use is Winfield Gaylord, who spoke of his desire to drain the 'capitalist swamp' that was DC. He was appropriating the language of land reclamation, which was popular in Fascist and (more importantly) Communist countries, and to some extent in democracies too. The phrase fell out of use in the postwar period. Reagan later used the phrase in 1983, which is why conservatives use it. Reagan claimed to have invented the phrase and it is notably different than the socialist version. If we take him at his word, that means it was independently invented twice.

It then continued as a basically bipartisan political catchphrase. Notably, it was used in the same year (2006) by both Pat Buchanan and Nancy Pelosi. Trump then popularized it even further and it has since been adopted by right-wing movements in other countries. And since then we have seen some people try to link that rhetoric directly to Nazism or Fascism, often with historically dubious claims that conveniently ignores that both political parties used the phrase for almost three decades. Trump's use only has a direct genealogy to Reagan. If Reagan's comment has any genealogy, it's to American socialism, but Reagan claimed he invented it on his own.

I think what you are seeing is that Hitler is a very ecological thinker. He thinks of the world in terms of land and its uses (specifically by races). So there's a lot of talking about geological features in general. I could comment on how Trump isn't but direct discussion of Trump probably breaks the twenty year rule.

Broadsides by Victor Burger, Pelosi says she would 'drain the swamp' from The Washington Post, Reagan Still 'Draining the Swamp' from The Chicago Tribune, The Reform Party from The New York Times, Adolf Hitler: Complete Speech Collection by Adolf Hitler, Hitler's Vision by Timothy Snyder.

You might want to check the book 'Male Fantasies' by Klaus Theleweit. It discusses Nazi imagery in detail with special emphasis on this very phrase, I'm sure he brings up examples from Nazi and proto-Nazi literature.