Sixty years before the Battle of Yavin (60 BBY), which is known officially as the "Rise of the Empire era" (spanning from 1000 BBY - 0 BBY) or, rarely, as the "Republic Classic era".
Following the end of the Mandalorian Wars in 3960 BBY, the loose reunification of the Mandalorian Clans under Mandalore the Preserver (AKA Canderous Ordo from KotOR I & II) ten years later, and the end of the Great Galactic War and the subsequent Cold War that followed 300 years later (see the upcoming MMO game Star Wars: The Old Republic), the Mandalorian culture had reverted to a corrupted state of what was once their calling in the Galaxy - warfare and conquest. The corruption manifested itself in the actions of the Mandalorians that acted out of "savagery", resorting to raiding and pillaging to get by in life besides that of mercenary and "muscle" work.
In 60 BBY, Jaster Mereel, upon becoming Mandalore, instituted a series of reforms known as the "Supercommando Codex", supercommandos being the elite of elite of Mandalorian martial traditions. These reforms aimed to reorganize the Mandalorians from the petty raiders and brigands they had degenerated to and restore the sense of honor and glory found in battle that was once their core and pride as a people. One Mandalorian, however, refused to acknowledge it, His name was Vizsla, and he would go on to recruit others who sought no more than to continue their ways of pillaging in spite of Mereel's codex. Vizsla would go on to found the Mandalorian Death Watch, which would then spark a civil war with the "True Mandalorians" that would rage for 16 years (44 BBY) until the Battle of Galidraan, which would go on to involve the Jedi Order (led by Jedi Master Dooku, no less), which, having been fooled by the Death Watch-backed planetary government into thinking the True Mandalorians (now under the leadership of Jango Fett, having taken over from Mereel following his death at the botched Battle of Korda VI at the hands of Vizsla after the ambitious Mandalorian Montross left a wounded Mereel on the battlefield after the latter came to dissuade the former from continuing his foolish frontal assault against the Death Watch-armed-and -trained Kordans - Montross was then thrown out by Fett very soon after) were butchering political activists. The Mandalorians were completely annihilated, save for Fett, who was thrown into slavery until he finally exacted his revenge against Vizsla on Corellia after a heated space and ground fight. You see, Vizsla not only killed his adopted father and mentor Jaster Mereel and nearly extinguished the Mandalorians from the Galaxy, but Fett's biological family on Concord Dawn was also executed by Vizsla's men during a brutal raid on the family farm (his father was a Journeyman Protector for the True Mandalorians).