The original Bulldog, however, was preserved by working class immigrants who brought their working dogs with them to the American South. Small farmers and ranchers used this all-around working dog for many tasks. By the end of World War II, however, the breed was almost extinct. Mr. John D. Johnson, a returning war veteran, decided to resurrect this breed. Along with Alan Scott and several other breeders, Johnson began carefully to breed American Bulldogs, keeping careful records and always with an eye for maintaining the breed's health and working abilities. The American Bulldog was recognized by the United Kennel Club on January 1, 1999.
Perhaps the most important role of the bulldog and the reason for its survival and in fact why it thrived through out the South was because of the presence of feral pigs, introduced to the New World and without predators[2]. The bulldogs were the settlers' only means of sufficiently dealing with the vermin. By World War II, the breed was near extinction until John D. Johnson and his father scoured the backroads of the South looking for the best specimens to revive the breed. During this time a young Alan Scott grew an interest in Mr. Johnson's dogs and began to work with him on the revitalization process. At some point, Alan Scott began infusing non-Johnson catch bulldogs from working southern farms with John D. Johnson's line creating the now Standard American Bulldog. At another point, Mr. Johnson began crossing his line with an atavistic Bulldog from the North that had maintained its genetic athletic vigor. This created a falling out between Johnson and Scott causing them to go their separate ways and breed the two significantly different versions of the American Bulldog.
Source: Wikipedia.org