World War I started in 1914 and lasted until 1918, and World War II was from 1939 to 1945. The United States entered the second World War in 1941. Major League Baseball players and teams supported the war effort by conducting demonstration trainings, opening stadiums for war-related charity events and participating in Liberty Loan bond drives. For American forces overseas, baseball was a means of boosting morale. The American military created 77 baseball diamonds in France, and on any given day some 200 games were played throughout the country. In May 1918, the Selective Service Division issued the “work or fight” rule, which stated that by July 1, all men of draft age not involved in “useful” work could be brought before a draft board and compelled to participate in war work or military service. The regulation came to include such service professions as elevator operators and doormen as well as those involved in games and sports. The onset of World War II created a significant shortage of professional baseball players, as more than 500 men left MLB teams to serve in the military. On January 14, 1942, MLB Commissioner Kenesaw Mountain Landis wrote a letter to U.S. President Franklin D. Roosevelt and pleaded for the continuation of baseball during the war in hopes for a start of a new major league season. With the approval of President Roosevelt, spring training began in 1942, this is referred to as the "Green Light" letter.