While seemingly a sensible limitation to free speech, the "shouting fire" line was coined to justify prosecuting a man who was protesting the draft. (This man argued that the draft was a form of involuntary servitude. I think he had a solid point.)

In a nutshell, it was argued that in this case the political speech represented a "clear and present danger" to national security.

Never concede to them anything. Any slack you give them, they will use to hang you.

> This man argued that the draft was a form of involuntary servitude. I think he had a solid point.

In a modern context, absolutely; most laws and treaties banning unfree labour specifically exempt military service, which is considered unfree labour by just about all authorities on the matter.