The background story of the main campaign is settled in a future universe. It's related to the setting created by Isaak Asimov in his novels of the Robot series. But, in my setting no robots appear and the conflict between Spacers and Terrans escalates.
2053: First permanent colony founded on mars.
2104: Mars II declared independence of the outer colonies from Terra.
2106: Independence war ended, when Terran forces destroyed Jupiter moon Callisto with a planet cracker bomb.
2124: Hyper drive engine was invented on Ganymede colony.
2125: First space flight to an other solar system was done.
2135: Aurora, the first colony outside the sol system was founded.
until 2200: 18 other colonies were founded in other solar systems.
2214: The Spacer League declared independence from the sol system. Aurora became the formal headquater of the Spacer League. After that, the terrans declared war and attacked several outposts of the Spacer League.
2217: The terran forces were defeated in the battle of Pollux IV. They lost about 80% of their space fleet. In the result, the terrans were banned in the sol system and the Spacer League became the new destiny of mankind.
2219: The remaining space forces of the Spacer League became the Space Guards. Their mission is to ensure the safe travel through space. They are the executive authority in space.
2614-Today: Free trading between the spacer worlds ensures wealth, the competition of products ensures research and development. This way, science advanced. The former colonies developed their own way of life.
There are three different groups in the known universe:
In the main campaign, you will be a Spacer, that was born on Aurora. Because you are being tired of lifing in a humdrum, sated life, you enlisted to the SpaceGuards. This is some kind of police or militia in the space between the Spacer worlds.
The campaign starts as you arrive at the Olympus station in orbit of Aurora. This station is the commando center as well as the central training center of the SpaceGuards.