What about this plot:
Before the Clone Wars Taris (a non-racist somewhat rebuilt form of the one in KOTOR) left the Republic. The people believed that the taxation and regulations placed on it by the Republic far exceeded the benefits of belonging to the Republic. When the Clone Wars began, however, they refused to join the CIS, believing it would be the same with them and wanting to remain neutral.
The CIS didn't like their refusal to join, and decided to try and "persuade" the leadership the join. When bribes, threats, and assassination attempts failed, they launched an invasion. They didn't count on the fact that the old ruins would provide ample cover for the locals, and faced far more resistance than they'd expected.