The 1940s were a time of significant change for baseball. The decade began with the starring performance of players like Joe DiMaggio and the emerging Ted Williams, who had Major League Baseball's last .400 batting average season in 1941. From 1942 to 1945, baseball was shaken by World War II. Over 150 players from the Japanese Professional Baseball League and the minor leagues lost their lives during the conflict. Many leagues shut down operations during the War, though Franklin D. Roosevelt encouraged the major leagues to continue functioning, which they did throughout the period. One unique league did start up during this period, the All-American Girls Professional Baseball League, founded in 1943, as women filled a professional baseball gap in the Midwest when so many male players went to War. In order to keep playing, major league teams had to look far and wide for ballplayers who were not engaged in the war effort. Those included players too young for the draft (Joe Nuxhall, Carl Scheib and Tommy Brown) or who were ineligible because of various physical ailments (one-armed outfielder Pete Gray was the symbol of that era, but many others had medical deferments). As a result, baseball's hierarchy was upset for a time, with the St. Louis Browns making the only World Series appearance in their history in 1944, the Washington Senators having a rare competitive season, and the Chicago Cubs reaching the Fall Classic for the last time in the 20th Century in 1945. Things returned to normal after players were released from their military obligations late in the 1945 season and came back en masse beginning in 1946.
After the War, there were a couple of major events. Jorge Pasquel and other Mexican League owners signed major league players like Max Lanier, Sal Maglie and Bobby Estalella, the top Cuban players of the era and some Negro League stars (the latter two groups had been active in Mexico for some time) to form a strong challenge to MLB supremacy. Legal entanglements arose and those white players who jumped contracts were blacklisted and barred from returning for a couple of years. The talent level was not as high as Mexico had seen in 1940-1941, when people like Josh Gibson and Cool Papa Bell were playing, but it has been the most-discussed era of the Liga.
Another key event was the signing by Branch Rickey of Jackie Robinson of the Kansas City Monarchs, a move that integrated baseball. Robinson debuted with the Montreal Royals in 1946 and four other ex-Negro Leaguers played in the Brooklyn Dodgers chain, most notably Roy Campanella. The move was opposed by many Southern fans and some leagues in the region never integrated, eventually leading to their collapse, most notably the Southern Association in the early 1960s. After leading Montreal to the International League title in 1946, Robinson made his Major League debut on Opening Day of 1947. While he encountered significant hostility, Robinson won the support of teammates and of Commissioner Happy Chandler, and by the end of the season, the American League had also integrated, with Larry Doby being the ground-breaker, while a number of other black players had made their Major League debut. The movement was so rapid that by the end of the decade, the Negro Leagues were on the verge of collapse, as all of its prominent players had been signed to contracts in organized baseball.
Many minor leagues were started and re-started after the War, creating a brief golden age for the minors which would be cut short in the dark age 1950s due in large part to television changing Americans' entertainment habits.
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-  Article on the effect of World War II on star batters' statistics in The Hardball Times.