what does "nothing save" mean?

No, it might have been better punctuated. You need to give us more context. This usage might be something such as:

He had nothing to work with, save his hands and his native intelligence.

Nothing save means nothing but what follows. So, ". . . nothing save a full sized rebellion . . ." means nothing but a full sized rebellion, nothing other than a full sized rebellion, etc., could save the empire from decay.

The meaning of 'nothing save' means 'what is required' or 'nothing but' or 'The absolute minimum action that should be taken is...'

nothing short of a full sized rebellion

Save can be used as a synonym for 'except' or 'excepting'.

nothing save a full sized rebellion, and the consequent reign of martial law, could save the empire from decay.

nothing except a full sized rebellion, and the consequent reign of martial law, could save the empire from decay.

The ship sank. All aboard save John were drowned.

I lost a number of keys. I found them all save one.

I'd punctuate it as:

Nothing, save a full sized rebellion and the consequent reign of martial law, could save the empire from decay.

"nothing except"

Wouldn't make sense; but doubtless you've already been so advised

Nothing but rebellion and martial law could save The Empire