In 3964BBY Mandalore the Ultimate called together the clans and began augmenting his armies further with Neo-Crusaders, his intention was to pull the Jedi Order into open conflict. As is the tradition in their culture, the Mandalorians sought to establish themselves as the most powerful faction in the galaxy by defeating the faction they considered to be their greatest rivals.
The Republic fought back but without the help of the Jedi. The war was going bad for the Republic, after many losses and against the order of the Jedi Council a splinter group of Jedi led by Revan and Malak joined the fight against the Mandalorian invaders.
The tide of the war shifted swiftly as the Mandalorians were driven back. In a final battle Revan single-handedly killed Mandalore the Ultimate and detonated a super-weapon blowing up a planet and killing many.
The ninth in a series of video entries giving the back story for the new Star Wars MMORPG The Old Republic. Watch the video below.
Most of the Knights of the Old Republic comic takes place in 3962BBY and 3963BBY.
I highly recommend reading the comics it's so good. Start with Star Wars Omnibus: Tales of the Jedi, Vol. 1.
Get your copy of Star Wars Omnibus: Tales of the Jedi, Vol. 1 here