Reaper War

The Reaper War was the catastrophic conflict that engulfed the galaxy beginning in 2186, when the Reapers finally arrived from dark space to carry out the harvest that had ended every advanced civilization in countless previous cycles. Hitting Earth and the Systems Alliance first, the Reapers swept across the galaxy faster than its navies could respond, falling on the turian and asari homeworlds and beyond. The war was the last chapter of the Mass Effect trilogy's story, and it drove old enemies into desperate alliance and centered on the effort, led by Commander Shepard and Admiral Steven Hackett, to complete the Crucible and end the threat for good. (Mass Effect 3)

The fall of Earth#

The Reapers took Earth in a matter of hours. The Alliance had expected the first wave to arrive from batarian space but was unprepared for its speed and scale. Rather than risk combat on emergence, the Reapers flew straight from relay to relay where they could neither be tracked nor intercepted, bypassing the Sixth and Seventh Fleets and overwhelming the others. The First Fleet, lying in wait at the Charon Relay, was destroyed, and the Fourth Fleet near Earth had only minutes of warning. (Mass Effect 3)

After destroying Earth's comm buoys, satellites, and undersea cables, the Reapers cut the planet's communications down to outdated radio towers and a handful of quantum entanglement links. Capital ships annihilated entire cities while destroyers descended to herd surviving populations, an act of cold efficiency rather than mercy meant to make the coming harvest easier. (Mass Effect 3)

The opening campaign#

The batarian homeworld Khar'shan fell next. The Reapers arrived in the Vular system, destroyed its communications, and within a day descended on Khar'shan, whose Hegemony mounted an uncoordinated response before the planet went silent. Refugees poured into human space in such numbers that Alliance officials at first mistook them for an invasion. (Mass Effect 3)

Turian space was struck through the colony Taetrus, chosen because it carried deep significance in the turian psyche as the site of the Vallum Blast. When the Turian Hierarchy sent a fleet in retaliation, Reaper ships ambushed it as it emerged from the relay. Faced with the choice of defending their side of the relay or pressing the attack, the turians continued the assault and were defeated. The fight for turian space had begun. (Mass Effect 3)

The Battle of Palaven#

When Taetrus fell, the turians massed in force around their homeworld, Palaven. Fleet Admiral Irix Coronati stationed two carriers near the system relay in what became known as the "Fifteen-Minute Plan," using swarms of unmanned fighters and drones to gather vital data on the Reapers' range, composition, and position as the enemy emerged. Coronati then made a short, daring FTL jump that landed his dreadnoughts in the middle of the Reaper fleet, turning the Reapers' great size against them so that the turian guns could lock first and down several capital ships. (Mass Effect 3)

The Reapers countered instantly, jumping destroyers into Palaven's skies to begin orbital strikes. Forced to defend the planet far from the relay against an endless line of Reaper ships, the turians suffered massive casualties before Coronati ordered a retreat. The turians insisted Palaven was not lost but that the battle had merely moved to the ground, where a heavily armed citizenry resisted the husk hordes even as Reaper capital ships destroyed city after city. (Mass Effect 3)

The fall of Thessia#

The assault on the asari homeworld Thessia did not go as smoothly as the Reapers' other strikes. Rather than meet the enemy head-on, the asari used dangerous hit-and-run tactics, blasting a Reaper ship and then jumping to FTL where they could not be tracked, forcing the Reapers onto the defensive. The Reapers' greater numbers let them accept these losses, however, and they soon ignored the harassment and began orbital bombardment, forcing the asari to defend Thessia in a traditional stance. (Mass Effect 3)

A swift and brutal slaughter of the asari ground forces followed. Resistance from trained biotics barely slowed the attackers, and Thessia's minimal military forces, combined with their unpreparedness before an overwhelming enemy, resulted in the fall of the planet. (Mass Effect 3)

The Battle of Rannoch#

As the war ground on, the quarians launched a campaign to retake their homeworld, Rannoch, from the geth, who had been upgraded by the Reapers. The opening engagement went badly for the quarians, whose Heavy Fleet was outmatched and forced to retreat at FTL speeds to rally with the Civilian Fleet behind Rannoch's sun. From hiding the quarians planned counterattacks to sever the geth's link with the Reapers, preying on a damaged dreadnought and sending strike teams to the surface to destroy the Reaper transmitting improved software to the geth. (Mass Effect 3)

How the battle ended branched on the decisions made in its final moments. In some accounts, an emergency order halted the fighting once the Reaper uplink was disrupted, and against the odds all three quarian fleets and the geth obeyed the ceasefire, leaving the two peoples to attempt coexistence on Rannoch and commit their combined armadas against the Reapers. In other accounts the quarians pressed the attack and were nearly annihilated when the geth, recovering their full capabilities, laid waste to the liveships and Civilian Fleet. In still others the quarians eliminated the Reaper at the decisive moment, crippling the geth command network and hunting the leaderless machines to destruction. (Mass Effect 3)

The Cerberus coup and the Miracle at Palaven#

While the great battles raged, Cerberus and the human councilor Udina attempted a coup on the Citadel, intended as a bloodless takeover that would grant Udina emergency powers to command the Citadel Fleet and liberate Earth. The plot unraveled when it was discovered, and the Illusive Man's assassins, led by Kai Leng, were dispatched to silence the councilors. The attempt failed and cost humanity its councilor; persistent rumors suggested Udina may have been indoctrinated, though no direct evidence was ever found. (Mass Effect 3)

On Palaven, the turians and krogan mounted a counterattack that combined deception, courage, and tenacity. The dreadnought Indomitable feigned a drive failure to lure the Reaper capital ships away from the planet, allowing turian transports to land krogan commandos who linked up with the resistance and delivered warp bombs and fission weapons. Smuggled inside Reaper processing ships, transports, and capital ships when those vessels opened to indoctrinate turian leaders, the explosives detonated in simultaneous strikes that returned large territories to turian and krogan control. The victory, won at the cost of the insurgents' lives and the civilians held within the destroyed processing centers, lifted the morale of the resistance and the wider galaxy. (Mass Effect 3)

The decisive battle#

The war was settled in a climactic battle over Earth, where the galaxy's assembled fleets engaged the Reaper armada while a strike force fought to reach the Citadel and activate the Crucible. The superweapon, built by a united galaxy and joined to the Citadel as its final component, offered Shepard a way to end the conflict, and its activation reshaped the galaxy in ways that branched depending on the choices made at the very end of the war. It closes the Milky Way chapters of the Mass Effect timeline, which then jumps six centuries to Andromeda. (Mass Effect 3; Mass Effect: Andromeda)