Battletech in 60 seconds:
First, we went into space. Then, we shot each other a lot. Then we kinda got together and formed the Star League. Then the Star League peeps shot the other peeps and took over, so everybody was happy and part of the Star League (well, almost everyone). There was, of course, more shooting of peeps here and there, during this time.
Then, there was a civil war, and the Star League wasn't so happy anymore. Kerensky said, "**** this ****, I'm out", and took most of the Star League military with him, and got the fudge out of Dodge.
While he was gone, everyone who used to be part of the Star League brained one another halfway back to the stone age, while everyone who went with Kerensky shot each other a bit, then advanced their technology further, then came back to the Inner Sphere a few hundred years later.
There was more shooting of peeps, the Clans conquered 20% or so of the Inner Sphere, then agreed to fight AT&T for control of some planet called Tukayyid in the *** end of nowhere. They didn't bring enough peeps to the fight, they lost, and they couldn't advance beyond that point for the next several years.
Then, when that deadline was coming up, the IS banded together, called themselves the Star League, and steamrolled Clan Smoke Jaguar, because they got tired of their orbital bombardment of civilian cities, and other such nonsense. After that, they fought another quick battle against the Clans, and made the Tukayyid treaty permanent. Clan Wolf ragequit, though, and said it didn't count.
Then the Word of Blake launched a holy war, pissed everyone off, and got pwned.
Did I miss anything big?