Youre correct that there was a purge of military in Nazi Era Germany, but beyond that you kind of misunderstand the very collaborationist relationship that the nazis and the existing military industrial apparatus. I would really suggest reading The Economy and Class Structure of German Fascism by Alfred Sohn-Rethel for a little bit of a contextualization of why what you’re saying is kinda surface level analysis. Separately I’d recommend Fascism and Social Revolution by R. Palme Dutt.
The US isn’t a democracy though, it is a dictatorship of the bourgeoisie, where every election we get to select which group of oppressors will wield state power against us until the next one.
Also decline is caused by anticolonial resistance and the logic of capitalism breaking down. I would suggest reading Lenin’s “imperialism” and Fanon’s “wretched of the earth”