From the website:
The great experiment that was the Republic of the Sphere has failed. Withdrawn behind the Fortress walls, the once-great power has become a silent, opaque remnant of its former glory. Without its influence, old hatreds have risen anew. As war once more rages across the Inner Sphere, new equipment strides across ancient battlefields. Technology, once stagnated by trade restrictions and peace treaties, now surges forward again, testing these new machines in the fierce crucible of war.