Complex sentence: His defence also falsely claimed that the Italians were under no orders to fight from the War Office in Brindisi, and would therefore have to be regarded as mutineers or "franc-tireurs" who had no right to be treated as POWs under the Geneva conventions.
Split into simpler sentences: His defence also falsely claimed that the Italians were under no orders to fight from the War Office in Brindisi. Therefore, they were mutineers or "franc-tireurs" who had no right to be treated as POWs according to the Geneva conventions.