The New Sith Wars

"During the New Sith Wars, the Mandalorians would wage war against both the Jedi and the Sith. They opposed the mysterious Dark Lord of the Sith known as the Dark Underlord, having cut a deal with the Jedi Master Murrtaggh to assault the Dark Underlord's camp in a diversionary role, while Murrtaggh sneaked into the fortress of the Sith Lord to slay him.

The Mandalorian distraction was successful: they engaged the Dark Underlord's Zeltron general and drew off his forces while the Jedi Master assassinated the dark side master, giving himself over to the dark side in the process.

A Mandalore during this era by the name of Ung Kusp was tricked into a war with the Sith; whether his war was the one against the Dark Underlord, and its result, are unknown.

Centuries later, Mandalorian armor was present at the Sith camp following the Battle of Ambria. Whether this meant that the Mandalorians had fought for the Sith at Ambria, that Mandalorians had attacked the Sith camp, or only that some Sith used captured or purchased Mandalorian armor, is unknown.

With the end of the wars, and the beginning of a thousand years of peace, the Mandalorians began to forget their warrior past, and, over the next few centuries, the Mandalorians slowly transformed into a less clan-based, more mercenary-oriented culture. "

-wookipedia