What makes the Umbara arc more painful than it already is? The Umbarans were originally with the Republic at the start of the clone war. They only left and joined the Separatists because their ambassador, Senator Mee Deechi was assassinated by another senator’s aide.
(Yeah, you probably remember this guy now. He represented Umbara in the Galactic Senate, and was later murdered by Lolo Purs.)
Now, that’s not an entirely unreasonable justification for a system to want to leave a governing body. The Republic couldn’t even protect the Umbaran representative from being killed by someone who was supposedly their ally. So the Umbarans seceded. And the Republic retaliated by invading their home planet.
Yes, Umbara was near some vital supply route or another. But the fact remains that Umbara was once a part of the Republic, and there is a strong indication they were specifically targeted because they left. The Umbarans weren’t the aggressors in this situation. They didn’t even have a standing army to speak of – they relied on local militias and Separatist droid supply ships to defend their planet.
So the clones had to go through all that trauma and death because, at best, the Republic desired a better supply route. At worst, because Palpatine wanted to show other systems what would happen if they left the Republic.
One last tidbit: the advanced Umbaran technology was appropriated by the Republic after they took the planet, and was later integrated into the Imperial military of the Empire.
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