First Clone War.

"To say we must co-exist with the Republic is just another way of saying we must kowtow to the Republic -- to place their values before our values; their traditions before our traditions; their culture before our culture... Compromise will inevitably lead to annexation, and then everything we are -- everything we ever were or could have been -- will be lost. No, I will not stand for this, and neither should you. Either the Republic falls or we fall; there can be no middle ground."

- Ranah Teh Naast, to the Mandalorian clans

The First Clone War, originally known as the New Mandalorian War, informally known as the Clone's War, was a fullscale war fought between the Galactic Republic and the Mandalorians from 84 BBE to 81 BBE.

Prelude
Prior to the First Clone War, the Mandalorian civilization had been at peace with the Republic for countless millennia. This all changed, however, when Ranah Teh Naast -- the insane clone/daughter of Uvhen Chal -- rose to power over the Mandalorians and crowned herself Mandalore the Destroyer.

An incredibly charismatic ruler, Mandalore the Destroyer managed to convince most of her people that they had grown stagnant and complacent in the thousands of years following the original Mandalorian Wars, that they were allowing their culture to die out through slow assimilation into the Republic. Sentiments of cultural pride and xenophobic fear kindled in their hearts and minds, the Mandalorians allowed Mandalore to lead them into a war against the Republic.

The event
Having grown complacent themselves over the millennia, the Republic didn't have a large-standing military force and quickly found themselves unable to adequately defend themselves when the Mandalorians invaded Republican space, leaving only the Jedi -- who did have a sizable army -- to defend them.

At first the Mandalorians were triumphant; not only was their army twice the size of the Jedi's, but they had managed to nearly cripple the Jedi by capturing and destroying many key shipyards and military installations. Mandalore's own unstable mind proved to be her own undoing, however, and she began making decisions which led to more-and-more losses to the Republic.

In the end, the Mandalorians found themselves facing destruction at the hands of the Republic, but Mandalore -- conceited and insane -- refused to make peace. Finally fed up, a faction of Mandalorian dissidents assembled and staged a coup, assassinating the insane clone and ending the war in the process.

Aftermath
Ultimately, there was to be no peace for the Mandalorians. With Mandalore the Destroyer gone, a power vacuum was left behind, and the Mandalorians -- those who had rebelled against her and those who had remained faithful -- became embroiled in a civil war. The Mandalorian Civil War eventually led to the devastation of their capital world and the fall of their civilization.