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.
- Mark Z. Aaron and Bill Nowlin, eds.: Who's on First: Replacement Players in World War II, SABR, Phoenix, AZ, 2015. ISBN 978-1-933599-91-5
- Steven R. Bullock: Playing for Their Nation: Baseball and the American Military during World War II, University of Nebraska Press, Lincoln, NE, 2004.
- Gene Fehler: Tales From Baseball's Golden Age, Sports Publishing LLC, Champaign, IL, 2000. ISBN 978-1582612478
- David Finoli: For the Good of the Country: World War II Baseball in the Major and Minor Leagues, McFarland, Jefferson, NC, 2002.
- Donald Honig: Baseball When the Grass Was Real: Baseball from the '20s to the '40s Told by the Men Who Played It, University of Nebraska Press, Lincoln, NE, 1993 (originally published in 1975). ISBN 0803272677
- Donald Honig: Baseball Between the Lines: Baseball in the '40s and '50s as Told by the Men Who Played It, University of Nebraska Press, Lincoln, NE, 1993 (originally published in 1976). ISBN 0803272685
- Bill James: "The 1940s", in The New Bill James Historical Baseball Abstract, The Free Press, New York, NY, 2001, pp. 197-219.
- Andy Jurinko and Christopher Jennison: Golden Boys: Baseball Portraits, 1946-1960, Skyhorse Publishing, New York, NY, 2012. ISBN 978-1616084509
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- Brent Kelley: The Pastime in Turbulence: Interviews with Baseball Players of the 1940s, McFarland, Jefferson, NC, 2001. ISBN 978-0-7864-0975-4
- John Klima: The Game Must Go On: Hank Greenberg, Pete Gray, and the Great Days of Baseball on the Home Front in WWII, Thomas Dunne Books, New York, NY, 2015. ISBN 978-1-2500-6479-0
- Bill Marshall: Baseball's Pivotal Era, 1945-1951, The University Press of Kentucky, Lexington, KY, 1999.
- William B. Mead: Even the Browns: Baseball During World War II, Dover Publications, Mineola, NY, 2010 (originally published in 1978).
- Bill Nowlin, ed.: Van Lingle Mungo: The Man, The Song, The Players, SABR, Phoenix, AZ, 2014, pp. 8-12. ISBN 978-1-933599-76-2
- Jeff Obermeyer: Baseball and the Bottom Line in World War II: Gunning for Profits on the Home Front, McFarland, Jefferson, NC, 2013. ISBN 978-0-7864-7043-3
- Philip Roth: The Great American Novel, Vintage Books, New York, NY, 1995 (originally published in 1973) ISBN 978-0679749066
- Bryan Soderholm-Difatte: The Golden Era of Major League Baseball: A Time of Transition and Integration, Rowman & Littlefield Publishing Group, Lanham, MD, 2015. ISBN 978-1-4422-5221-9
- Rick Swaine: The Black Stars Who Made Baseball Whole: The Jackie Robinson Generation in the Major Leagues, 1947–1959, McFarland, Jefferson, NC, 2006. ISBN978-0-7864-2316-3
- James D. Szalontai: Teenager on First, Geezer at Bat, 4-F on Deck: Major League Baseball in 1945, McFarland, Jefferson, NC, 2009.
-  Article on the effect of World War II on star batters' statistics in The Hardball Times.