The Anti-Federation League was a group of individuals who opposed the United Federation of Planets and the ideals and methods of the Federation Starfleet. They believed that, since Starfleet was primary an exploratory and diplomatic organization, starships should not be equipped with mass armaments and engage in battle with other species and empires.
In 2267, a group of League members, led by a man named Alan, departed on a two-month cruise into Klingon space aboard the SS Harmony, in the hopes of achieving what Starfleet and the Federation couldn't do, and make peace with the Klingon Empire. Unfortunately, their efforts resulted in their ship being attacked and them being taken prisoner. Eventually, four of the six were rescued alive by a team from the USS Enterprise. (TOS - Errand of Fury novel: Demands of Honor)