In my personal opinion I feel the Jedi should mobilized early on and should have just cut off the threat at it's source. The source being Mandalore. After all, the Jedi had numerous infiltrators (shadows in the game) that could have probably killed Mandalore early on and prevented most of the bloodshed. Also the Jedi should have recognized the threat, they say they are recovering from previous wars, well if they stopped the war before it started, they would have more time to recover before the Sith returned.
Jedi aren't assassins. And they aren't galactic enforcers. Unless these systems asked for help, the Jedi and the Republic couldn't do anything. Obviously they didn't have time to ask. Its a flawed system, but the Jedi were bound by Republic law.
Like I said, the law was flawed. Which led to the Republic trying to get as many systems into the government as possible to avoid something like this happening again.