The 18th Deep Space Fleet is actually the military service of the Tri-Planet Coalition (TPC). The reason it's called the 18th is that every time technology necessitates the replacement of battle vessels, the fleet is renamed chronologically. Hence, there have been 18 fleets in service with the TPC since its inception. The TPC consists of the planets of Tiber, Incron, and Hondus listed from smallest to largest. Planetary colors are displayed in the 18th's standard; three flags of dark blue (Tiber), black (Incron), and yellow (Hondus). Able to harvest the resources of three worlds has given the 18th vast capabilities in technology and weapons.
One sub-faction exists in the TPC, the Death Guard. Formed of veterans of the 18th or adventurers, they are supplied with weapons and vehicles out of the 18th's armories; they even have a small task force consisting of ten ships. There have been reports of another sub-faction within the 18th known as the PLAGUES, these soldiers are mainly made up from workers turned into vigilantes, they are checked up every so often by the Death Guard and reports state that they are working with the 18th to combat the COP.
The TPC was first introduced to the humans of Earth when a strange vessel appeared in the Centauri system and claimed to be from a land called Canada on a distant planet. They sent the humans back on their way with promises that they would one day meet again. When the clouds of intergalactic wars darkened on the horizon, the TPC was approached by envoys of both the Coalition of Planets and the United Nations of Earth to join their sides. The COP was stronger, but the UNE was more peaceful, along the lines of the beliefs of the TPC. Realizing that they would either have to stand together with someone or stand alone sooner or later; the 18th allied itself with their brother humans on Earth, sickened by the genocidal desires of the COP. To read a deeper history, and details of the individual planets, visit: The TPC Wiki