After discovering nuclear weapons, the krogan cold war goes hot, plunging their homeworld into nuclear winter and reverting society to a clan structure.
As the first species to discover mass effect technology and FTL travel from Prothean ruins, the asari inevitably become the first to find the Citadel, a massive space station of unknown origin.
After salarians become the second species to discover the Citadel and open diplomatic relations with the asari, the two races are poised to dominate galactic politics in the foreseeable future.
As the asari and salarians begin to make first contact with new species, they establish a new government, its capital the Citadel, with themselves as the only members of its Council.
Aftering opening a mass relay, the Council discovers the rachni, a race of intelligent hive-minded insects. Negotiation being impossible, a war inevitably breaks out.
After wiping the rachni extinct, the Council grants the krogan a new world to settle. Free of their harsh homeworld, the krogan population explodes uncontrollably.
Shortly after making first contact with the krogan, the salarians "uplift" them with mass effect technology, recognizing their robust physiology as ideal for fighting the rachni.
In order to accommodate their growing population, the krogan begin aggressive expansion into other species' territories, prompting retaliation from the Council.
With no way to sustain a war after the genophage's deployment, the krogan are eventually forced to surrender. Their role as the Council's military and peacekeeping force is replaced by the turians.
Shortly after the Council makes first contact with the turians, the krogan launch a preemptive strike against their colonies with WMDs. The turians quickly join the Council in the Krogan Rebellions as a result.
In order to prevent the krogan from replenishing their forces, the turians deploy a salarian-engineered bioweapon called the genophage, which reduces the viability of krogan pregnancies to one in one thousand.
For their role in the Krogan Rebellions, the turians are granted a seat on the Council alongside the asari and salarians, the first race other than its founders to be granted this honor.
The quarians, a technologically gifted people, inadvertently create a race of AI, the geth, as they gradually upgrade the machines they use for cheap labor. The geth rebel against their creators, driving the quarians from their worlds and forcing the quarians to become a nomadic people. However, the geth do not expand beyond their creators' former territories.
After centuries of pollution and failing to achieve spaceflight, the drell homeworld is primed to crash, eventually leading to the extinction of the drell. However, the hanar make first contact with the drell at this time and evacuate some 375,000 drell before the population collapse begins.
Shortly after discovering the existence of mass effect technology from Prothean ruins on Mars, humanity learns of a mass relay encased in Pluto's moon Charon. This relay is activated and humanity begins interstellar travel.
The trilogy's protagonist is born.
Humans make first contact with the turians while trying to activate a dormant mass relay, who open fire as this violates Council law. The Council intervenes before conflict escalates into full-scale war, and humanity is welcomed into the galactic fold.