The Old Sith Wars

"Almost three thousand years after the conquest of Mandalore, the Mandalorians wandered the Galaxy far from their home systems hoping to begin a new crusade against worthy foes. This brought them to the Empress Teta System during the period when the Krath had gained control of the system.

The Mandalorian clans became allied with the Sith after their leader, Mandalore the Indomitable, was bested in combat by the fallen Jedi turned Sith Lord, Ulic Qel-Droma. Mandalore pledged loyalty to Qel-Droma who in turn was apprentice to Dark Lord Exar Kun and in their name the Mandalorians laid waste to countless worlds in the years that followed, gaining the enmity of the Republic, and, most crucially, the Jedi.

Ten years after the defeat of Exar Kun, dozens of Mandalorians were unjustly arrested and executed after a psychotic bounty hunter named Jigger Wraith preyed upon innocents using rare Mandalorian weaponry. It was during these times that the Mandalorians would reform under Mandalore the Ultimate and begin constructing a large assortment of forces from their scattered forces. They would reconstruct defenses on Dxun as well as other Mandalorian-held worlds in order to build a mass stockpile of equipment and vessels. They would eventually be approached by the Sith and persuaded into attacking the Republic for the honor of facing "a worthy foe".
Many species comprised the Mandalorians, such as this Rodian.

The Neo-Crusaders were an assortion of many diffent races led by Mandalore the Ultimate and started the Mandalorian Wars. During this time, the Mandalorians would cause chaos upon the Republic, committing atrocities such as wiping out most of the Cathar race, and reducing the surface of the planet Serroco to glass with nuclear bombs. The Mandalorians' victory seemed assured, until the Jedi Knights Revan and Malak led a large portion of the Jedi Order to war, no longer able to sit back and watch the Republic crumble under the Mandalorians vicious attacks. The Mandalorians were then slowly but surely pushed back to the Outer Rim, where Revan lured them to Malachor V. Revan personally killed Mandalore the Ultimate in close combat, and utterly destroyed the Mandalorian Fleet, along with a large part of the Republic's as well.

After their defeat in the Mandalorian Wars at the hands of the Jedi Revan and Malak, the Mandalorians went into a long decline. Revan destroyed their stockpiles of weaponry and battle droids, and exiled the clans into the Outer Rim. One group settled on Tatooine, but apparently lost contact with other Mandalorians over time.

Some Mandalorians were unable to accept the terms of their defeat, and turned to piracy, brigandage, and mercenary work – e.g. Sherruk's group, who became bandits and troubled the settlers of Dantooine in 3,956 BBY. Canderous Ordo, who fought side by side with a redeemed Revan against Darth Malak during the Jedi Civil War, later claimed the rank of Mandalore and led a small remnant of his followers on the moon of Dxun. It was again Ordo, who traveled the galaxy with the Jedi Exile, and gathered the scattered Mandalorians who became mercenaries. Ordo was also a key in defeating the Sith Lord Darth Nihilus on the battleship Ravager."

-wookipedia