"A degree in krogan history" was apparently once a joke in academic circles, but excavations on Tuchanka show that ancient krogan had a thriving and impressive civilization. Unfortunately, their technological advances ran afoul of krogan aggressive instincts. Once the krogan entered the atomic age, they didn't hesitate to deploy nuclear weapons on Tuchanka, devastating the surface and reducing their people to scattered warring bands.
The salarians technologically and culturally uplifted the krogan, in return for their service against the rachni who were terrorizing Citadel space. Capable of surviving the harsh environments the rachni preferred, krogan units drove the rachni to extinction, and were hailed as heroes. They were granted gentler worlds to colonize, but their rapid breeding meant the krogan soon overran them and threatened neighboring species.
The resulting Krogan Rebellions lasted almost a century, until the turians deployed the genophage bioweapon. Unable to breed reinforcements, the krogan people fell into decline.