Once the humans started looking towards the stars again with renewed enthusiasm in the early 2160s, after a very short radio signal was picked up that could only have been made by a transmitter, their efforts to make space travel possible accelerated drastically. Soon they founded a colony on their Moon, and shortly thereafter on Mars.
They were determined to find the source of the signal, correctly believing it to be signs of sentient life on other planets. By the year 2283, inter-planetary travel was a commonplace, and humans even had space stations orbiting moons of Saturn. With the help of their mastery of gravitational slingshots, traveling between Saturn and Earth took only a few months instead of years.
The peace among the governments of Earth, however, could not last forever, and a third world war broke out that threatened humankind. The war between the European Commonwealth, the United Nations, The Red Star Alliance, and the United States of America lasted nearly fifty years, and cost millions of people their lives.
Humanity was then divided into three factions - the Sol Alliance, the Titan Regime, and the Martian Conglomerate. Each faction sought to further their own cause, and this selfish hatred for each other led to nearly five hundred years of scientific and technological standstill, until the Sol Alliance forced their competitors to form the United Planets and act as their vassals. The people of the United Planets were treated like glorified slaves, but they saw themselves two different governments forcibly unified under the Sol Alliance banner.
As they expanded, the Sol Alliance started granting the United Planets more and more freedom in order to keep order and peace, but tensions were still high. The Sol Alliance and the United Planets were constantly bickering, and even engaging in short firefights from time to time. Neither side wanted to admit defeat, or declare war, so the pattern continued for many years. Humans quickly regained much of their lost time, and by 3005 most of the reachable Sol System was colonized. The Sol Alliance assembled the largest fleet in human history - hundreds of thousands of ships housing billions of people. The fleet, dubbed the Sol Alliance Terra (SA Terra), used gravitational slingshots to reach light speed, which they now know is Warp Velocity.
Their original destination were Proxima Centauri and Sirius, however as they traveled, they got caught in a wormhole and ended up in the Orion Nebula. The United Planets immediately started colonizing habitable worlds while the Sol Alliance tried to study wormholes and set up Warp Gates and borders. It took almost two years for most signals to reach the Sol System, so the colonists were mostly independent. They started disliking the lack of proper support from Earth, and soon formed a new faction: the Terran Empire. As they had no idea where exactly they were, they settled on terrestrial planets and started exploring the surrounding systems in smaller groups.
The Sol Alliance did not take kindly to this treachery, and since the Terran declaration of independence, the Sol Alliance send yearly fleets to bolster their still loyal United Planet and Sol Alliance colonies. The ongoing civil war threatens to rip the human's governments at the seams, but they are, as always, too selfish to care. With the founding of the small interstellar republic, the Terran Empire drew attention to themselves and were contacted by the Crale in 3013. The Crale immediately offered them an alliance pact, quickly recognizing their vastly superior weaponry. They offered humans more efficient starship designs, as well as the promise of future planets, in return for the human's promise to protect them from outside harm.
The Sol Alliance is ruled by a Supreme Chancellor that has an entire council of ministers that rule different aspects of the republic. The council is funded and essentially ran by corporations; ministers usually get to their office by bribing and blackmailing their way up the food chain, meanwhile a corporation can "sponsor" a minister and pay for his campaign in return for the minister's promise to put the corporation's needs first. The corporation that funded the current Supreme Chancellor might as well say they run the Sol Alliance.
The United Planets is the opposite; it's ran by a Senate without a true leader, who vote on everything. It's very diplomatic and democratic in all the senses of the word, as any lowly citizen can be voted on and off of the Senate. This ensures they always have the people's best interest at heart.
Gravitational Slingshots work by vessels firing electromagnetic waves at nearby celestial bodies, and using the pull of it to rocket past.
Once the ships reach Warp Velocity, physical distance and time matter little - they are able to cross a thousand light years within a single year.
Warp Gates allow a fraction of warp velocity to be reached and allow ships to cross a light year within a month.