Complex sentence: The Army Transport Service (ATS), originating with the Quartermaster Corps in 1898 and continuing into Transportation Corps as a division, operated the Army's large ships, most of which were transports, but ATS also manned the Army's large cable ships.
Split into simpler sentences: The Army Transport Service (ATS), originating with the Quartermaster Corps in 1898 and continuing into Transportation Corps as a division, operated the Army's large ships. Most were transports, but ATS also manned the Army's large cable ships.