Starsiege is up there with 40k in terms of places you don't want to live as the average joe. After the events of Starsiege, the already oppressive empire is driven to desperation, barely surviving against the Cybrids. However, mega corporations begin to become much more influential, and they prove to just as big as dicks as the the empire, and create Bioderms, which are essentially cyborg slaves. For the outskirts of the empire, things were bleak as supplies dwindled.

The Bioderms, under the leadership of an unknown figure, rebel against the empire and the megacorps and throws things into chaos. Some isolated colonies, already beyond imperial space in whats known as the 'Wilderzone' pretty much broke all ties to the empire and fractured entirely into competing entities known as tribes and united under a group known as the Children of the Phoenix, a tribe who worshiped Harabrec Weathers, a major imperial knight who resisted the empire.

The empire, struggling against the Bioderms and pissed off at their major traitor being worshiped, tried to send some knightly orders on a crusade to put them in their place, but this failed, leading to the eventual creation of the  Blood Eagles, rogue knights who are extremely brutal and hates pretty much anything that moves. Differences in culture lead to the creation of the Star Wolves, who in turn hated the Beagles for their many, many atrocities, and were dissatisfied with the Children of the Phoenix's "my way or the highway" stance on everything, and chose to roam the Wilderzone. The Children of the Phoenix began to lose their control over the tribes, and things fell to chaos for centuries. Chaos with very little happening, mind you, aside from the Beagles and the Star Wolves killing each other.

The Diamond Sword literally came out of nowhere under the leadership of "the Enlightened Master" and remained neutral mostly, preferring to meditate and be vague. On the whole, they don't do much and keep a low-profile.

Tribes 2 has the Scourge, part of the rebelling Bioderm armies, enter the Wilderzone to loot and pillage. Aside from some lore, this doesn't really show up in the game itself.

Tribes Vengeance is a prequel to the first tribes game, and highlights the nature of the Empire's involvement, the beginning of the Blood Eagles, and just how the Children of the Phoenix set up their own demise later.

Tribes: Ascend is the newest foray, and focuses (so far) on a new conflict between Beagle and Diamond Sword, for hitherto unknown reasons beyond a betrayal and other tensions.

Ironically enough, Tribes was envisioned as a small scale tactical shooter, with a commander mode and a heavy base element, but a physics glitch known as skiing that let players go fast eventually turned it into a fast-paced game about speed and offense.