The baseball color line, sometimes called the "Gentleman's Agreement", was the policy, unwritten for nearly its entire duration, which excluded African American baseball players from organized baseball in the United States before 1946. As a result, various Negro Leagues were formed, which featured those players not allowed to participate in the major or minor leagues.

Origins
On May 28, 1916, Jimmy Claxton temporarily broke the professional baseball color barrier when he played one game for the Oakland Oaks of the Pacific Coast League. Claxton was introduced to the team owner by a part-Native-American friend as a fellow member of an Oklahoma tribe. Within a week, a friend of Claxton revealed that he had both African American and Native American ancestors, and was promptly fired. It would be nearly thirty more years before another black man played organized white baseball.

Jimmy Claxton
The Negro National League was founded in 1920 by Rube Foster. This created two parallel major leagues, and until 1947, professional baseball in the United States was played in separate homogeneous leagues.

The Negro National League
During his term in office as the first baseball commissioner, Kenesaw Mountain Landis has been alleged to have been particularly determined to maintain the segregation. It is possible that he was guided by his background as a federal judge, and specifically by the then-existing constitutional doctrine of "separate but equal" institutions (see Plessy v. Ferguson). He himself maintained for many years that black players could not be integrated into the major leagues without heavily compensating the owners of Negro League teams for what would likely result in the loss of their investments. In addition, integration at the major league level would likely have necessitated integrating the minor leagues, which were much more heavily distributed through the rural U.S. South and Midwest.
One disincentive to desegregation was that strong players who were white sometimes threatened to quit any team that took on a black player.
Although Landis had served an important role in helping to restore the integrity of the game after the 1919 World Series scandal, his unyielding stance on the subject of baseball's color line was an impediment. His death in late 1944 was opportune, as it resulted in the appointment of a new Commissioner, Happy Chandler, who was much more open to integration than Landis was. Social change was in the wind, as the U.S. Military had become largely integrated during World War II.
From the purely operational viewpoint, Landis' predictions on the matter would prove to be correct. The eventual integration of baseball spelled the demise of the Negro Leagues, and integration of the southern minor leagues was a difficult challenge.

Kenesaw Mountain Landis
In 1943, baseball executive Bill Veeck attempted to buy the Philadelphia Phillies franchise; rumors began circulating that he intended to purchase the contracts of several Negro Leaguers in order to make the longtime also-rans more competitive in a period when war requirements had depleted most rosters. However, the franchise was instead sold to a different ownership group, and some historians have recently questioned the likelihood of Veeck's rumored intentions.
Around 1945, Branch Rickey, General Manager of the Brooklyn Dodgers, held tryouts of black players, under the cover story of forming a new team called the "Brooklyn Brown Dodgers". The Dodgers were, in fact, looking for the right man to break the color line.
In his autobiography, Veeck, as in Wreck, Veeck stated that he wanted to hire black players for the simple reason that in his opinion the best black athletes "can run faster and jump higher" than the best white athletes. Veeck was seldom known for being politically correct, and that kind of comment would now be considered an outdated stereotype. Rickey was not known to have made any similar remarks, nor was he known to have had an especially liberal world view. The broader view of both men is simply that they were businessmen who saw a large pool of talent that they wanted to utilize, if at all possible. The more important point that Veeck uncovered (and presumably Rickey also) was that there was no actual rule against integration; it was just an unwritten policy, a "Gentlemen's Agreement".

Bill Veeck and Branch Rickey
The color line was formally breached when Branch Rickey, with the support of the new baseball commissioner, Happy Chandler, signed the African American player Jackie Robinson in October 1945, intending him to play for the Brooklyn Dodgers in the National League. After a year in the minor leagues with the Montreal Royals (International League), Robinson endured epithets and death threats and got off to a slow start in his first major league season in 1947, but his athleticism and skill earned him the Rookie of the Year award.
Less well-known was Larry Doby, who signed with Bill Veeck's Cleveland Indians that same year to become the American League's first African-American player. Doby, a more low-key figure than Robinson, suffered many of the same indignities that Robinson did, albeit with less press coverage. Both men were ultimately elected to the Baseball Hall of Fame on the merits of their play. Due to their success, teams slowly but surely integrated talented African-Americans on their rosters.
Prior to the integration of the major leagues, the Brooklyn Dodgers spearheaded the integration of the minor leagues. Jackie Robinson and John Wright were assigned to Montreal, but also that season Don Newcombe and Roy Campanella became members of the Nashua Dodgers in the class-B New England League. Nashua was the first minor-league team based in the United States to integrate its roster after 1898. Subsequently that season, the Pawtucket Slaters, the Boston Braves' New England League franchise, also integrated its roster, as did Brooklyn's class-C franchise in Trois-Rivières, Quebec. The rest of the minor leagues would slowly integrate as well, including those based in the Southern United States. The Carolina League, for example, integrated in 1951 when the Danville Leafs signed Percy Miller Jr. to their team.

Jackie Robinson and Larry Doby
The Boston Red Sox were the last major league team to integrate, due to the steadfast resistance provided by owner Tom Yawkey and manager Mike "Pinky" Higgins. However, they too eventually conformed to the integrationist trend, signing Pumpsie Green in 1959.