The krogan are a species of large reptilian bipeds native to the planet Tuchanka, a world known for its harsh environments, scarce resources, and overabundance of vicious predators. The krogan managed to not only survive on their unforgiving homeworld, but actually thrived in the extreme conditions. Unfortunately, as krogan society became more technologically advanced so did their weaponry.

Four thousand years ago, at the dawn of the krogan nuclear age, battles to claim the small pockets of territory capable of sustaining life escalated into full scale global war. Weapons of mass destruction were unleashed, transforming Tuchanka into a radioactive wasteland. The krogan were reduced to primitive warring clans struggling to survive a nuclear winter of their own creation, a state that continued until they were discovered by the salarians two thousand years later.

With the help of the salarians, the krogan were 'uplifted' into galactic society, and lent their numbers and military prowess to bring an end to the Rachni Wars (see below). Ironically, after the rachni were eradicated, the rapidly-expanding krogan became a threat to the galaxy in turn, starting the Krogan Rebellions and forcing the turians to unleash the genophage. This genetic 'infection' dramatically reduced fertility in krogan females, causing a severe drop in births and, ultimately, population, eliminating the krogan numerical advantage.


-Snipped from http://masseffect.wikia.com/wiki/Krogan