Complex sentence: At that time, all the islets of the Kinmen Archipelago were considered as war zones under Martial Law, which was to allow the Republic of China (Taiwan) to prevent an attack by the People's Liberation Army on the Communist Party of China.
Split into simpler sentences: At that time, all the islets of the Kinmen Archipelago were war zones under Martial Law. This was to allow the Republic of China (Taiwan) to prevent an attack by the People's Liberation Army on the Communist Party of China.