From The Starfield Wiki
The United Colonies or UC are the largest military and political power in the Settled Systems. Based out of their capital city of New Atlantis, the United Colonies help maintain the current status quo in space. If the melting pot of New Atlantis is any indication, the United Colonies maintains a cosmopolitan society.  In the year 2310 AD, the United Colonies went to war with their primary rival for power, the Freestar Collective. This conflict would become known as the Colony Wars. The war ended with an uneasy peace being established that has lasted until the present day, 2330 AD.