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As the Dwarfs with Thorgrim, I convinced Zhufbar to Confederate. However, confederation meant that all their holds had their garrisons reset to 0. As a result, before I could get an army raised there, an Orc army was able to easily capture Zhufbar, which had a garrison of units each of which now only had 4 dwarfs.
While the garrison reset makes sense if a hold is changing hands due to conquest, it really makes no sense when it's due to confederation.