Within Blake's holy writings the Word of Blake was supposed to join the Star League and help bring the IS into a golden age. When the Star League dissolved the WOB turned its massed forces on hidden worlds, forces build in the FWL, and others on the Great House homeworlds. Tharkad was hit immediately, New Avalon, etc. Pretty much holding a gun to everyone's head, telling them to reform the Star League. The WOB was preparing to drive out the Clans and that is why their Shadow Divisions were such a horrendous threat. They had been finding adrift ships, refitting old ones, and building new ones and prepared a massive force. Some people don't think it rational that the WOB had such strength, I could definitely see a fanatical, super secret society working continuously to build up. Anyway, the WOB was taking over and killing people all over the IS. Then, Devlin Stone led a rebellion on Kittery that gained enough steam to get support from the Houses and finally build up a coalition to turn the tide and start driving the WOB back.
After this, Stone took the portion of space that was roughly a similar area to the Terran hegemony and created the Republic to be that beacon of hope that the IS no longer needed war as the WOB showed that even this far in the future, people could still succumb to power hunger and use all manners of evil to kill others. He began disarming individual units, lords, and other small time forces. The Houses gave a show of good faith in some instances and reduced some forces while mothballing others. In the end, it wasn't going to last because "war...war never changes". (fallout)
The HPGs drop, everyone panics. Industrialmechs start becoming front line units mounting missile launchers and autocannons. The Republic fractures and the prefectures wind up splitting off toward their home factions. Battlemechs are appearing more, they are brought out of storage, museum pieces are used in desperation. Now, everywhere is in full fledged war. The Wolves and Falcons dug deep into the Lyrans. Dracs and caps are hitting FedSuns. The Republic just brought down their wall and are about to surge out into the IS in an attempt to retake their lost worlds.
Things are about to get very exciting.