The Reunification Alliance is a large and powerful collection of planets and civilisations, bound together by the Reunification Alliance Charter and under the mutual protection of the Alliance Central Authority. The Alliance holds full-member status in the League of Interstellar Civilisations despite its illegitimate status in the eyes of a number of Non-aligned Worlds.
Despite initially operating with a joint-capital system, Terra, also known as Earth, is now firmly established as the Alliance capital, with many of its most prominent institutions based there, or elsewhere in the Sol system.
The Alliance is a spacefaring power that places a strong emphasis on trade, exploration, and colonisation. This is perhaps natural, given that planetary affairs are left almost entirely to the discretion of local planetary governments (so long as they abide by the charter), leaving the Alliance itself to focus solely on outward-looking commitments in the great black.
Further Reading
Governance of the Reunification Alliance
History of the Reunification Alliance
Reunification Alliance Culture
Reunification Alliance Economy
Alliance Central Authority
Alliance Astrotime Authority
Reunification Navy
Alliance Combat Forces
The Nightwatch