"Some one hundred years before the Battle of Yavin, the leader of the Mandalorians was assassinated by the bounty hunter Durge. The Mandalorians struck back, and inflicted terrible tortures on the bounty hunter, but he managed to escape and entered into hibernation to heal his wounds.
In 60 BBY a schism occurred in their ranks. A violent splinter group, known as the Mandalorian Death Watch, revolted against the leadership of Jaster Mereel, who led the True Mandalorian loyalist faction, along with his Brother's adopted son, Rotok Scarr, due to Mereel's proposed Supercommando Codex reforms.
Jango Fett leading the Mandalorians at the Battle of Galidraan.
Eventually the civil war between these groups spilled over to the pastoral world of Concord Dawn. Here the Death Watch leader Vizsla attempted to murder Jaster. However, thanks to the assistance of a young orphan named Jango Fett, Jaster and his followers escaped the trap. Eventually, Vizsla caught up to Jaster and the Mandalorians on a distant world and, due to the treachery of fellow Mandalorian Montross, Jaster Mereel was killed in battle.
Jaster's protégé, Jango, took over the reins of leadership, despite Montross's disparaging comment that it would mean a "child leading them." Jango's tenure as Mandalorian leader ended at the Battle of Galidraan, where a Jedi task force under the leadership of Master Dooku led to the death of every remaining Mandalorian, except for Jango. Dooku remembered Jango and later employed Jango to be the Prime Clone after Dooku left the Jedi Order.
During this time, Mandalorians of an unknown faction massacred Ryyk's family.
After this disaster, the Mandalorians were effectively ended as a fighting force in the galaxy. Individuals survived on both Mandalore and in other isolated pockets of known space, but they played little influence on galactic affairs until the Clone Wars, when Jango Fett became the template for a Grand Army of the Republic on Kamino. "
-wookipedia