After the Rachni War, the quick-breeding krogan expanded at the expense of their neighbors. Warlords leveraged their veteran soldiers to seize living space while the Council races were still grateful. Over centuries, the krogan conquered world after world. There was always "just one more" needed. When the Council finally demanded withdrawal from the asari colony of Lusia, krogan Overlord Kredak stormed off the Citadel, daring the Council to take their worlds back.
But the Council had taken precautions. The finest STG operators and asari huntresses had been drafted into a covert "observation force", the Office of Special Tactics and Reconnaissance. The Spectres opened the war with crippling strategic strikes. Krogan planets went dark as computer viruses flooded the extranet. Sabotaged antimatter refineries disappeared in blue-white annihilation. Headquarters stations shattered into orbit-clogging debris, rammed by pre-placed suicide freighters.
Still, this only delayed the inevitable. The war would have been lost if not for first contact with the turians, who responded to krogan threats with a prompt declaration of war. Being on the far side of krogan space from the Council, the turians advanced rapidly into the lightly-defended krogan rear areas. The krogan responded by dropping space stations and asteroids on turian colonies. Three worlds were rendered completely uninhabitable.
This was precisely the wrong approach to take with the turians. Each is first and foremost a public servant, willing to risk his life to protect his comrades. Rather than increasing public war weariness, krogan tactics stiffened turian resolve.
The arrival of turian task forces saved many worlds from the warlords' marauding fleets, but it took the development of the genophage bioweapon to end the war. There were decades of unrest afterwards. Rogue warlords and holdout groups of insurgents refused to surrender, or disappeared into the frontier systems to become pirates.