When humanity started colonizing other planets, the Terran Alliance was giving out starships, some people ran further than most, instead of 'second star to the right and straight on to morning.'
About 1600 light-years due rimward of Terra, past the Twilight nebula, lies what many of its inhabitants refer to as the Outer Zone. This area is literally filled with hundreds of habitable worlds, giving the impression of the Taurian Concordat’s Hyades Cluster writ large. The region is largely peaceful, the technological advances are considerable, but the various interstellar nations are militarized to a degree that would seem insane to spheroids. In addition, the major reason for the peace is neither the Ares Conventions, nor the similar New Geneva Conventions signed by the OZ nations, but something much older: Mutually Assured Destruction. It is for these reasons that they can boast something the rest of the Inner Sphere cannot: They were never part of the Star League.
Ironically, the primary reasons for their militarization were found back in the Inner Sphere. In the chaos after the Outer Reaches Rebellion, the combination of heavy and obvious militarization across the Inner Sphere, the use of military force to suppress internal dissent, the perception that the military is less accountable to the citizens and is becoming more involved in the political process, and a total and utter disregard for collateral damage caused many people to have an understandable reaction: 'Let's get the hell out of here!' The militarization and violence across the Inner Sphere easily allowed people wanting to leave the Inner Sphere behind and find somewhere to start a new life to be funneled towards the OZ. Many of these people entered the OZ at the New Geneva system, where they later joined one of the existing nations, or more rarely, started their own.