Complex sentence: It also annexed Katanga, a territory held under the Congo Free State, which Leopold had gained in 1891, when he sent an expedition that killed its king, Msiri, cut off his head, and hoisted it on a pole.
Split into simpler sentences: It also annexed Katanga, a territory held under the Congo Free State, which Leopold had gained in 1891. He had sent an expedition that killed its king, Msiri, cut off his head, and hoisted it on a pole.