The story behind 2300 AD is that after a nuclear war which is the basis for the RPG Twilight 2000, humanity has reconstructed the earth and is heading out into space. However, the nation-state remains the dominate form of political organization, and the dominant power is the French Third Empire.
Faster than light drives called stutterwarp are available in this universe, working by making a great many small faster-than-light jumps, one after another. However, they cannot be used for very long distances, needing a discharge at a gravity well after a few light years.
This has caused a definite pattern to colonies and exploration, with colonised space being divided into a number of 'arms', or chains of nearby stars. There are three main ones, called the Chinese Arm, the French Arm, and the American Arm. Only a handful of alien races have been discovered.