The Krogan Rebellions were a major conflict between the krogan and the Citadel Council that began in 700 CE and continued for decades. The Rebellions were preceded by over three hundred years of aggressive krogan expansion coupled with booming population growth. Tensions came to a head when the krogan remorselessly advanced onto worlds already claimed by other races. With the krogan unwilling to back down despite Council demands, the Rebellions ensued.
The krogan became more aggressive as other races tried to protect their worlds, until the krogan attempted to settle the Asari Republics world of Lusia. When told by the Council to leave, the krogan refused. Their representative, Overlord Kredak, stormed out of the Council chambers, daring them to take their worlds back. The war began soon after.
The Council had taken precautions. The finest STG operators and Asari Huntress bands 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 sudden explosions, headquarters stations shattered into orbit-clogging debris, rammed by pre-placed suicide freighters.
One early success took out the fuel collectors at Nith. Spectre agents managed to insert a virus into the computers of the solar power arrays; every fifth array suddenly applied braking thrusters. The arrays behind them “piled up”, and all were reduced to wreckage. The krogan never had the resources to rebuild the arrays, depriving them of their fleet’s main fuel supply for the remainder of the war.
Overlord Kredak was defeated over Niagolon, in the Tomaros System. As his flagship was discharging its drive core and undergoing repairs, an asari commando team snuck on board and detonated resonant warp bombs, sacrificing their lives to destroy the vessel through explosive decompression.
The krogans' incredible reproductive rate, however, left them the clear advantage.
The Council found an ally in the disciplined military forces of the Turian Hierarchy.
The Battle of Syphax was a meeting engagement between roving krogan warbands and the turian taskforce en route to Krogan-occupied Lusia that prematurely escalated into the largest space battle of the middle-phase Rebellions. While a massive Allied victory (one of the few since the SPECTRE raids on Arlakh and Nith), Syphax was retroactively considered strategically inconclusive at best and a setback at worst, as it caused a huge delay in the invasion schedule of krogan-held asari space; time that was used by the krogan to entrench themselves in those systems. Had the Turian Expeditionary Force been allowed to continue unimpeded, doubtless they would have dealt a more decisive blow to the unprepared defenders that might have brought a swifter end to the war.
Being on the far side of krogan space from the Council, the turians advanced rapidly into the lightly-defended krogan rear. 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 Hierarchy citizen is first and foremost a public servant, willing to risk his life to protect his comrades. Rather than increasing weariness for war, krogan tactics stiffened turian resolve.
Clan Vynchar were responsible for a number of high-profile massacres of turian civilians including the Quarasi Massacre and the sack of Dorea. Such actions earned them the ire of the Hierarchy military who pursued the Vynchar with particular zeal, eventually laying siege Battlemaster Vynchar Sorval’s fortress on the Terminus planet of Laena and orbitally bombarding the site for several days until the fortress and its subterranean bunkers were reduced to dust.
Information on Palus obtained by Clan Cruack spies led battlemaster Cruack Grenk to believe the colony was a relatively-undefended sanctuary for the families of a number of high-ranking turian military leaders. Hoping to crush the turian spirit by assassinating the families of their leaders, Grenk led the majority of his clan on a raid on the planet, only to discover that it was a trap laid by the turians. The entire colony was abandoned and empty, and as the invading Cruack forces attempted to flee, a turian cruiser hidden behind the planet’s moon annihilated the colony with a redirected asteroid.
A krogan warlord named Graken Dhal took the fight to Palaven's home cluster, the Apien Crest, bombarding the modestly-defended Digeris. When reinforcements came to intercept him, Dhal's navy put their rear to Digeris so that stray shots would hit the planet. The turians won despite this severe handicap, using countless fighters and cruisers to take down the krogan dreadnoughts. The 79th Flotilla, most notably, sacrificed themselves in great numbers to bring down krogan vessels attacking the colony.
The world of Cannis was seen by the krogan as a potential launching point for any assault on Palaven, and was subsequently conquered by Clan Dinroth in the First Battle of Cannis. Given its strategic value, Cannis was fortified heavily by the Dinroth, the clan moving almost all of their forces from Tuchanka to reinforce it.
The eventual turian counterattack, the Second Battle of Cannis, became one of the bloodiest of the entire Rebellion. Initially, both sides hoped to preserve the world’s infrastructure and environment for their own use. As such, traditional engagements were fought across the world’s surface with very little use of artillery or orbital weaponry. The Dinroth’s numerous fortified positions and hidden bunkers resulted in what has been compared unfavorably to Earth’s ‘trench warfare’, with hundreds of thousands of turian soldiers landing on the moon to face smaller but heavily-fortified krogan units. After several weeks of grinding assault, and with krogan reinforcements on the way, the turian General Trebonian ordered a withdrawal and bombarded the colony from orbit.
See: Borunian Massacre.
Deployment of the Genophage
The genophage's modus operandi wasn't to reduce the fertility of krogan females, but rather the probability of viable pregnancies: many krogan would die in stillbirth, with most fetuses never even reaching this stage of development. Moreover, every cell in each krogan was infected, to prevent the use of gene therapy to counteract it. The genophage was originally developed by the salarians as a deterrent to continued aggression. They believed that, as the consequences would be so devastating, no one would ever deploy the genophage, and its simple existence as a threat would cause the krogan to stand down. However, the salarians and the turians had different military attitudes, the turians only appreciating an approach of massive retaliation. They deployed the bioweapon on all krogan worlds and occupied territories.
The Third Battle of Uresium is widely held to be the turning point of the Rebellions, and was the first Citadel offensive since the deployment of the genophage. Uresium, which by that time had been contested for nearly a decade, inflicted the most severe defeat upon the krogan up to that point; a devastating loss from which they never recovered. Psychological operations were key to the victory - large numbers of krogan simply gave themselves up when promised a cure to the strange affliction that had been sweeping through the ranks. Of course there was none, and what followed next is a matter of controversy - the prisoners were rounded up, left in large open spaces, and bombarded from orbit. This method of disposal of surplus krogan military personnel was used on a few of the liberated asari and salarian colonies, where there were often too many prisoners to process, until the push began into krogan space proper and the practice was discontinued.
The krogan themselves were splintering by this point. In the closing years of the Rebellions, the five clans working the planetoid Durak, in the krogan home system, fell to fighting over a particularly rich deposit of iridium. All five warlords agreed to a Crush (a meeting at a neutral location) to negotiate a truce. Unfortunately, all five arrived planning to betray and destroy their fellows. While the leaders and their seconds met, all their bases were destroyed simultaneously by hypervelocity cannon strikes. Left with only the food, water, and air in their hardsuits and no way to call for rescue, the warlords apparently fought each other to the death. The survivors of the five “Durak clans” on Tuchanka still argue about which clan’s warlord was the last one standing.
A female krogan warlord, Shiagur, used her rare fertility to draw the strongest males to her band, but even she couldn't hold out against turian peacekeeping forces, who finally killed her at the Battle of Canrum. In retaliation, many of the turians responsible were hunted down and killed by vengeful krogan males. Canrum was the last krogan push before the Rebellions ended.
Finally, besieged on all sides and knowing the survival of their species was at stake, the krogan surrendered. That was not the end of hostilities, as many rogue warlords and insurgents refused to surrender, and had to be wiped out by turian task forces. Some even vanished into frontier systems to become pirates.
The Citadel Conventions were drawn up in the wake of the Rebellions, as a response to the destruction of the conflict and an attempt to distance the Council from the brutal krogan warfare.