The Mandalorian Empire, simply shortened to Mandalorians, was once a vast empire ruled by leaders called Mandalore. The origins of the empire date back several thousand years before they came under the rule of Exar Kun. This empire was clan-based and consisted of members of various species. The Mandalorian Empire was defeated at the end of the Mandalorian Wars and their empire torn apart. Almost a decade later, Canderous Ordo took the position Mandalore and brought the clans together under one banner, thus reorganizing the Mandalorian Empire. The Mandalore Empire fell rule to Jaster Mereel next. Under Jaster's rule, the empire prospered and became known in the galaxy.
The Mandalorian Empire sought out an alliance with the Jedi during Darth Hawken's campaign against the Republic and their Jedi allies. The alliance never happened but the Mandalorians did aid in the galactic effort to defeat the Sith tyranny. They fought and won against Sith forces on Ord Radama. They subsequently gave it to the Republic to care for. The Mandalorians would go on from being a great asset to the Republic to being public enemy number one.
The Second Mandalorian Wars were started by the Mandalorians. Everyone knows it, including the Mandalorians, but they needed a second great crusade and the Republic offered it. The Republic ships on Yavin IV were a prime target and Mandalore took the opportunity. The Mandalorians won the battle and the concurrent First Battle of Coruscant. The first half of the war went their way but they lost their edge when Operation: Crossroads was ended. The end of the war brought a new Mandalore, Rikenn Tarbann and a new alliance, the Grand Alliance.
The Galactic Sith Purge followed the Second Mandalorian Wars. It was a conflict between the Grand Alliance (the Republic, Jedi, and Mandalorian Empire) and the Kaidius Dynasty of the Sith Empire. The Grand Alliance was totally victorious with no loss at all. Many credit the Mandalorian Empire for such a perfect victory. When the Galactic Sith Purge was officially called to an end, the Mandalorians Empire were not too happy. Their leader was assassinated and the fingers were pointed at the Republic. The Sith are the real ones at fault, however.