BattleTech is another universe, a universe much better off for mankind than the Rifts universe. It is the 31st century, and mankind has spread to the stars. 3 centuries of almost continuous warfare have destroyed much of the technology that was developed, though much of it has been recovered in later years. The main means of warfare still happens on planets, using 30 meter tall robot vehicles known as BattleMechs, or Mechs. Vaguely humanoid in appearance, they weigh anywhere from 20 to 100 tons, and are capable of mounting a fearsome array of missiles, autocannons, lasers and particle cannon.
The period of time immediately following the invention of the jump drive, the only means of traveling between stars in this other universe, was more peaceful than humanity had known before. The Star League was formed, and was actually quite effective. That all changed in the 28th century, when a usurper to the throne of Star Lord effectively single-handedly demolished all that the Star League had sought to build. Alexander Kerensky, leader of the Star League Defense Force, stormed the citadel of the usurper with his forces. He then took most of the SLDF and left the Inner Sphere, the region of inhabited space. He said he left to prevent the SLDF from being used in the times of chaos that would follow. Sadly, his prediction was correct, and the Star League was torn asunder by the various Lords. The five Lords formed five nations; the Draconis Combine, the Federated Suns, the Lyran Commonwealth, the Capellan Confederation and the Free Worlds League. These Successor States warred among themselves constantly for the next 3 centuries or so, only occasionally pausing.
Many things have happened in the 31st century, and many books have been written about them. The point in history that this conversion is concerned with is 3058. Kerensky's SLDF returned to the Inner Sphere as the almost alien Clans, a people who had become almost solely adapted to warfare. They claimed that the Inner Sphere was their legacy, and that they sought to recreate the Star League. The invasion lasted several years, and was excessively bloody in terms of life lost. Only by banding together did the Successor States manage to stop the Clan invasion. Unable to continue their campaign to retake Earth, the Clans turned their focus inward, warring among themselves. With the threat of the Clan invasion gone, the Inner Sphere has likewise resumed warring among itself.
Because of how jumpships work, some of these 31st century soldiers have turned up on Rifts Earth. Jumpships work by tearing a hole in the universe and creating an artificial wormhole. The problem is that unless the drives are carefully charged, misjumps can occur. These ships and their crew are never seen again. This happened to the forces of three of the Clans, along with several mercenary armies. The jumpships appeared on Earth, coming through various rifts. The jumpships were without exception destroyed, as they were never made to operate in a gravity field, but the majority of the MechWarriors and their Mechs survived. Now, they must adapt to this new world.
I still have to get this from KJ.