The Rachni Wars were a series of conflicts that took place in the history of the Mass Effect series. Around 1 CE, a group of Salarian explorers discovered the Maskim Xul star system where they made first contact with the insectoid race known as the Rachni. The Rachni captured the expedition and reverse-engineered the Salarian vessel's systems in order to build FTL-capable starships of their own, thus enabling them to rapidly expand across the Milky Way galaxy.
The races of the Citadel attempted to make negotiations with the Rachni, but could not establish lines of communications with the Rachni Queens that dwelled deep beneath the surface of Suen. The Rachni swept through the galaxy like a plague, their numbers rapidly increasing with every planet they conquered.
For almost a century, the Rachni continued their march across the galaxy and crushed all resistance. The Salarian Union took drastic steps to try and drive the Rachni back by "uplifting" a new species, the Krogan. Like the Rachni, the Krogan were capable of rapid reproduction and could withstand most hostile environments, making them the perfect warriors to strike back at the Rachni where they lived. The Krogan were able to strike at the queens in their lairs and quickly reclaim conquered Council worlds. When Krogan fleets pushed the Rachni back to their homeworld Suen, the Rachni refused to surrender. The Krogan response was swift and brutal: they bombarded Suen's cities and detonated powerful bombs in the Rachni's underground nests, obliterating them and creating massive sinkholes on the planet's surface. The Citadel Council would normally have objected to such total destruction, but after centuries of relentless war, complete eradication of the Rachni seemed the only possible solution.
In 300 CE, the Rachni were declared extinct, although in caution the Citadel Council left a listening post in the Rachni's home system to monitor for any survivors. As a result of the Rachni Wars, the Council established new policies prohibiting the activation of any dormant mass relays that led to uncharted systems in order to prevent other disastrous encounters with undiscovered hostile races.