Complex sentence: Keintpuash and his group were the last captured on June 4,1873 when they voluntarily gave themselves up, after assurances from the U.S. government personnel that their people would be treated fairly and that all of the warriors would be allowed to live on their own land.
Split into simpler sentences: Keintpuash and his group were the last to be captured on June 4,1873, when they voluntarily gave themselves up. The U.S. government personnel had assured them that their people would be treated fairly and that the warriors would be allowed to live on their own land.