Not to be confused with the Alliance to Restore the Republic, an alliance dedicated to the overthrow of the Galactic Empire; or with the Galactic Federation of Free Alliances, the successor government to the New Republic.
The Unified Galactic Alliance was founded in 2184 when seven worlds, the Human worlds of Alderin, Coruscant, and Earth, the aarkash world of Idsocaa, the Wookiee world of Kashyyyk, the Nuzzalari world of Nuzzalar, and the Twi'lek world of Ryloth banded together to unify some 20,000 other systems under their leadership. Each of those systems was already a member of a lesser alliance of worlds and thus the union initially formed a large collective. The first president was elected shortly after and the Senate was established with the Alliance Council being made up from members of the 6 founding worlds.
Over the years the spread and influence of the Alliance spread and acquired new members, adding new seats to the Senate and several new members to the Council. The Space Police and Star Command were both incorporated into it during this early stage for law enforcement and protection, respectively. Also in the early years of the Alliance the council extended a hand out to the Jedi Council who agreed to work with the Alliance but remain a separate entity. By 2350 over 50,000 worlds were part of the Unified Galactic Alliance and the Alliance comprised the single most massive collection of star systems in the Milky Way galaxy.
Many worlds, though they are encompassed as part of Alliance territory do not necessarily count themselves as part of the Alliance. Some are in fact unaware that there is life in the universe beyond their world. These worlds are generally left to themselves until they achieve space flight, at least in theory. The Alliance, however, takes it upon itself to protect these worlds and if necessarily intervene in interplanetary conflicts.