starting with napoleonic tactics and ending with rolling artillery barrages and tanks.
Strictly speaking, just like in any war, they've just forgotten\ignored most of wisdom present in all wars since Sargon the Great. As humans always do.
Specifically, say, human wave attacks at entrenched positions with machine guns were not a thing of somebody not understanding the results. There were plenty of works written at that point about what warfare should be, it's just that their authors wouldn't get promotions in peacetime. Actual commanders had plenty of lives at their disposal without any particular responsibility for those and needed to report some successes. Existential wars between European nations were a forgotten thing, they were either not existential or against a technologically outclassed enemy, so having such people in charge was fine until WWI itself.
But that's not what I meant, I meant that WWII has that peculiar trait that one of the sides (the one that started the show) was actually properly organized before gaining experience through many deaths. Like 3 critical rolls in a row in DnD, one can say. That's how they managed to wreak such havoc.
I know that actual contact use of cavalry was obsolete.