Complex sentence: It is known that the liner was operating as a U.S. Army transport--under the name USAT "Kroonland"--by February 1918, when she was loaded with war materiel and departed New York for Saint-Nazaire, France.
Split into simpler sentences: It is known that the liner was operating as a U.S. Army transport by February 1918. Loaded with war materiel, USAT "Kroonland" departed New York for Saint-Nazaire, France.