World War I and Major League Baseball-- Military and War Plant Workers

From Baseball Reference.

World War I shut down the minor leagues of baseball one by one and caused Major League Baseball to shorten its 1918 season, ending it at Labor Day, Monday, September 2nd.

Notable players who missed playing time because of military service in World War I included Hall of Famers Grover Cleveland Alexander, Red Faber, Jud Wilson and Harry Heilmann.  A number of players also worked in wartime-related industries as War Plant Workers, particularly in steel mills and shipyards, and also missed playing time as a result.

War Plant Workers worked in industries supporting the war effort and were thus ineligible for the draft.

--Cooter