Ok.. to give a brief timeline.. about 500 years before when the main battletech stuff takes place.. there is a huge rift in the Inner Sphere politics. What was a peaceful era turned to the Houses wanting to be the strongest and making war against each other. The neutral "police" force, the Star League was lead by Alexander Kerensky. He saw what was happening and when he realized he couldn't stop it, he led what remained of the Star League out of the inner sphere.

For 500 or so years, nobody in the inner sphere knew what had become of them, but they had formed themselves into the clans. After that time, the clans came back and invaded the inner sphere. There are 2 types of clans, Wardens and Crusaders. The wardens believe they are meant to come in, integrate with the inner sphere and protect humanity. The crusaders believe they need to conquer the inner sphere in order to rule them to protect humanity because they believe the inner sphere are too corrupt to do it themselves, even with help. The first clan to take Terra (earth I guess) would become the primary clan that all others would have to follow.

During the clan invasion, there are a few remnants of the star league left in the inner sphere who are now called comstar, they had been the communications branch of the star league, but they kept what little original technology from back then. No place else had mechs as advanced as what they had once had other than comstar. They challenged the clans to a trial and if the clans lost, they would have to adher to a 15 year truce where they could invade towards Terra no further until the time was up. The clans sense of honour made them unable to disregard a battle challenge or the consequences thereof.

Ok that wasn't quite so brief but.. necessary lol. The Refusal War comes during that time of truce. Clan Wolf had let an inner sphere warrior captured in battle into their warrior caste and he made his way to become the leader, the Khan of clan wolf. By this time, half of Clan Wolf was warden, have was fanatical crusader. The warden half split off and left for the inner sphere. In respond to this, the falcons, who are also quite fanatically crusader, issued a trial of refusal.. basically a challenge because they disagreed with the actions of clan wolf. At this time, wolf and falcon were the two strongest clans among the remaining.. 17 or 18 (originally had been 20 clans). These two clans also had a very strong rivalry and the actions of the wolves made it worse. Both clans were quite decimated by the refusal war.