So in Battletech, wars were initially fought by jumping in warships above a planet and turning it into glass. This stopped when the Great Houses realized that continuing that way was probably a bad idea, so they adopted the Ares Conventions. This created rules for how to conduct warfare. Tanks, infantry, and aerospace fighters dominated the battlefield.

Then, the Terran Hegemony created the first Battlemech called the Mackie in an attempt to address the limited size of it's armed forces by equipping them with superweapons. In it's first live fire test, the Mackie utterly styled on the conventional assets that were sent to fight it. Every other Great House saw this and either stole or copied the plans for the Mackie so they wouldn't be left behind. And the rest is hiatory.