Following the United States's entry into World War I, the submarine protected Allied shipping lanes to the European countries. Departing New London, Connecticut, on 27 November, L-3 sailed for the Azores via Bermuda arriving Ponta Delgada on 13 January 1918. One month later, she was dispatched to the British Isles for patrol duty out of Berehaven, Bantry Bay, Ireland, to protect Allied shipping losses from U-boat attacks.
