Complex sentence: One of his most noted accomplishments during this period was the negotiation of peace between Chōshū and Satsuma, two powerful provinces that had long been hostile to each other, and then unite them against the Bakufu, the government that supported the Tokugawa shogun.
Split into simpler sentences: One of his most noted accomplishments during this period was the negotiation of peace between Chōshū and Satsuma, two powerful provinces that had long been hostile to each other. He then united them against the Bakufu, the government that supported the Tokugawa shogun.