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occupare to me as the sense of seizing (taking over) land, not actually seizing someone. I think the word you want is apprehendi (to be seized) from apprehendo, -ere. Also possible is rapi (to be snatched, seized) from rapio, -ere.
Also, it's barbaros apprehendi. Remember that iubeo takes the accusative/infinitive construction. And the infinitives are passive, not active: to be seized, not to seize. Note apprehendi in my explanation above.