How can I understand "could have been reached had Nazi discipline?"

Question

While I was reading a book, I found a sentence incomprehensible. It reads, "A danger point could have been reached had Nazi discipline and its totalitarian control been relaxed."

I can understand the meaning only vaguely through the context, though, still I'm not sure what the "had Nazi discipline" is meant to be.

And what's the exact grammatical structure of it?

Thanks!