Frustrated by the slow progress of the ACS, Tappan and a sizable nucleus of men, including his brother Arthur, Theodore Dwight Weld, Gerrit Smith, Amos A. Phelps, and James Gillespie Birney, left the ACS to join what was to become known as the "immediatist" camp, who wanted to end slavery in the United States (US). Weld gained considerable influence following the move of the Tappan brothers to this group. In December 1833, at Philadelphia, Lewis Tappan joined activists such as William Lloyd Garrison to form the American Anti-Slavery Society.