United Nations of Terra
Terra's countries tried to keep their sovereignty for as long as they could. But in the end, the leaders realized they could not keep a hold on their people and colonies at the same time alone. What was at first the name of their alliance became the global government, a parliament of representatives from all over the world and a major player in the Dominium's highest spheres.
The proud nations of Terra had a hard time reaching an agreement. They had a similar organism before, but the UNO was no good as a global leader and acted more like a meeting place that sometimes helped those in difficulty. It had too many loopholes and deadlocks that anyone could abuse to block the entire system, and was already deprecated by the time a world organisation was needed. With the emergence of new powers in other dimensions, the budding Dominium was about to fall out of their grasp. Furthermore, The I.D.E.A. was more powerful than any individual country in terms of economy, political influence and military, so a union was required to face what most leaders view as a direct threat to their power.
Putting petty matters aside, the leaders met directly in the ancient UNO parliament and discussed for days to form this new alliance. By the end of the meeting, only 7 of them refused to join the newly formed United Nations of Terra, U.N.T. for short. Although Nations of the United Terra was suggested in an early draft, it got swiftly rejected and only traces of this proposition remain. The decision to merge every existing government was not an easy one and came with lots of heated discussions and two wars, but worked out in the end.
It was decided that every former country would now elect not a president/monarch/emperor, but rather a representative to the U.N.T. in whatever way they see fit. Then the assembly would edict laws and the mutualised administrations and armies would apply them. This system was somewhat clunky and slow at first, but once the united rule was in place and the individual powers greatly reduced, people quickly forgot what were the countries they used to live in.
First rate ruler
With time, countries faded and got ultimately replaced by districts during the Great Restructuration. Boundaries were erased, some territories got split up into multiple districts and others merged together. The system remained unchanged though, except that now representatives are elected by each district, making them slightly over 500 people.
This gamble was a winning one however since the U.N.T. managed to take the lead of the Dominium's Directory. The thousands of worlds encompassed in the giant empire are ruled essentially by the U.N.T. and the I.D.E.A., and a swarm of small actors, mainly unions of colonized dimensions and smaller scale companies. The Terrans were the first ones to spread into the dimensions, and they are determined to ascertain their dominance.