The Unified Galactic Alliance
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 wookie 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. 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 comprises the single most massive collection of star systems in the galaxy.
Many worlds though they are encompassed as part of Alliance territory do not necessarily count themselves as part of the Alliance, some in fact are unaware that there is life in the universe beyond their world, these worlds are generally left to themselves till they achieve space flight. The Alliance, however takes it upon itself to protect these worlds and if necessarily intervene in conflicts between worlds.