Wikipedia: African Union

The only African state that is not a member of the African Union is Morocco, which left the AU's predecessor, the Organization of African Unity (OAU), in 1984, when many of the other member states supported the Sahrawi nationalist Polisario Front's Sahrawi Arab Democratic Republic.^[4][5] Morocco's ally, Zaire, similarly opposed the OAU's admission of the Sahrawi Arab Democratic Republic, and the Mobutu regime boycotted the organisation from 1984 to 1986.^[6] Some countries have since retracted their support for the Sahrawi Republic.^[7]