I've been reading a book about the history of the pulp novels - it's got chapters on all of the usual characters, like The Shadow and Doc Savage, but it also talks about some of the forgotten heroes. Most of them are forgotten because they were either cheap knock-offs or just plain schlock, but a few sounded pretty interesting, like Operator 5.
The premise of the stories is that an invading army rolls through the US in the early 30's and locks the country down under martial rule. The military is destroyed, the government is dissolved (the president shoots himself), and the only hope for salvation is a small band of freedom fighters led by Operator 5, the last remaining government agent.
Like a lot of the pulps, these stories were a weird mish-mash of current events, future speculation and crazy sci-fi which lead to some very strange ideas and imagery. Operator 5, the wholesome hero, wore the death's head skull as his symbol (pre-dating the SS), blew up buildings and bridges and regularly used his girlfriend as bait for enemy. I guess they didn't call it hard boiled action for nothing: