The Tarkis Hegemony was founded by former Dominion forces,
lead by Admiral Chrisitan Cromwell and his collaborators.
The forces rebelled against the policy of Earth when it was clear that personal freedom was at risk.
Ironically this lead to the total militarization of Earths society, because the Hegemony was threatening everything the people of Earth were working for. The Hegemony received help from varies colonies, that had suffered much under Earth's rules.
Most of its citizens are former soldiers or freedom fighters who believe that Earth lost its way.
Lead by the Admiralty Board and the Peoples Assembly, they consider themself the guardians of freedom. Because their society tries to provide their citizens with as much freedom as possible, its also a haven for smugglers and pirates.
They are at war with the Terran Dominion.