The Rachni Wars took place around 2100 years before the setting of Mass Effect. It started when the species on the Citadel were trying to expand Citadel Space. They foolishly opened a Mass Relay without any knowledge of where it led. They ended up opening a route to a system controlled by the Rachni. This hostile species tore through Citadel space and attacked anything in their sight. For nearly a century the Citadel races fought the Rachni and never gained an advantage, they were over-powered and out-numbered by the insect race.
During the war, the Salarians made first contact with another species called the Krogans. The Krogan home planet had been ravaged by a civil war and nuclear weapons. The Salarians helped the Krogan by giving them advanced technology and moving them from their irradiated planet to a safer location. The Salarians weren't doing this out of the kindness of their hearts. It was all part of a plan to groom the Krogan into fighting for the Citadel Council against the Rachni.
The Krogan join the Rachni Wars 80 years after it started and for the first time, the Citadel species had the advantage. Unlike other Citadel races, the Krogan were able to survive in the harsh environments on the Rachni home planets so they could attack the Rachni at the heart of their colonies. For 220 years the Krogans dominated the war until the year 300 CE when the Rachni were officially declared extinct.
After the war the Krogans were declared an official Citadel race and were given an embassy on the Citadel. The Krogan Monument was erected in the Citadel Presidium to commemorate the contribution of the Krogans during the Rachni Wars. The council also made it illegal to activate any primary Mass Relay without knowing where it leads to prevent another incident like the Rachni Wars.
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