Firstly, the conflict with the Vasudans had settled quite a bit by the time the Shivans show up. More akin to the level of conflict of Kilngons/Federation circa the Original Star Trek. Openly hostile, but not going out of the way to blow each other out of the stars. Besides, as I recall, there had only been a couple of destroyers lost during the whole war 14 years at the point where we come in.
Comparing to the NTF rebellion, as a military action, forgetting the behind the scenes politics, it felt like a mostly military matter. Bosch took those of his Command who were loyal and took 3 systems and kicked the GTVA out. We don't know how bad things were under his rule. We don't know if there were stormtroopers in the streets or if people got up every morning and went to work and lived normal lives with no change in their day to day activities. Also, background activities aside, the Alliance had painted the entire conflict with the NTF as a Liberation of those systems, there is no way they could have nuked the planets they were liberating and ever have expected to stay themselves or keep their flunkies, minions or co-conspirators anywhere near the positions of power they needed to enact they're plans.
Look, you guys have got me talking like Terran command is a bunch of 3rd class comic book villains now.