Complex sentence: Captain Chamier received intelligence of al-Shallash's activities and in response, arrested Deir ez-Zor's mayor on suspicion of collaboration with al-Shallash and on 10 December requested support via telegraph from British regional headquarters in Albu Kamal and Baghdad.
Split into simpler sentences: Captain Chamier received intelligence of al-Shallash's activities and in response, arrested Deir ez-Zor's mayor on suspicion of collaboration with al-Shallash. Furthermore, on 10 December, Chamier requested support via telegraph from the British regional headquarters in Albu Kamal and Baghdad.