In 1845, Alexander Cartwright led the establishment of the Knickerbocker Base Ball Club, the first team believed to have played baseball under modern rules. The club's by-laws committee formulated the Knickerbocker Rules, which in large part dealt with organizational matters and laid out rules for playing the game. In 1857, sixteen New York area clubs, including the Knickerbockers, formed the National Association of Base Ball Players (NABBP), the first organization to govern the sport and to establish a championship.
Aided by the Civil War, membership grew, leading to a more unified national version of the sport. Beginning in 1869, the association permitted professional play, addressing a growing practice that had not been permitted under its rules to that point. In 1870, a feud developed between professional and amateur ballplayers. The NABBP split into two groups, one became the National Association (NA), largely considered to be the first major league.