The 1918 major league baseball season began on April 15, 1918. The regular season ended earlier than originally scheduled, September 2, because of a reduced schedule due to American participation in World War I. The National League and American League champions were the Chicago Cubs and Boston Red Sox, respectively. The postseason began with Game 1 of the 15th World Series on September 5 and ended with Game 6 on September 11. The Red Sox defeated the Cubs, four games to two, capturing their fifth championship in franchise history, since their previous in 1916. Going into the season, the defending World Series champions were the Chicago White Sox from the 1917 season.