From the back of the book:
Invaders from Beyond
they came from beyond the Periphery: invaders with advanced weapons, strange tactics and stranger customs. Arriving in 3049, they attacked world after world with unrelenting fury. By 3052 they had conquered more than 200 planets, and their target was clear. They sought to capture the cradle of humanity: Terra. If not for the battle of Tukayyid and the resulting truce, the Earth would surely have fallen.
We now know these invaders are the Clans, the sons and daughters of the legendary Aleksandr Kerensky, gone from known space for over 250 years. They seek to overthrow the Inner Sphere and recreate the lost Star League in the own image...or so it seems...
Invading Clans describes the history, culture and military capabilities of Clans Smoke Jaguar, Nova Cat, Ghost Bear, Steel Viper and Diamond Shark. Also included is an overview of occupied worlds, deadly new BattleMechs, and eight full-color pages featuring renderings of Clan uniforms and ceremonies.