May 1: Earth
The Reapers bypassed the Sixth and Seventh Fleets at Terra Nova and Eden Prime, flying straight from relay to relay where they could neither be tracked nor intercepted. The tactic was unexpected, since the navies of organic species would never risk coming out of FTL within combat range, or leaving enemies at their backs to threaten supply lines. At Arcturus Station, more than a dozen Reaper capital ships engaged the Alliance’s Second, Third, and Fifth Fleets. This was mere screening for the main force. Dozens more capital ships continued through the Charon Relay, where the First Fleet had been lying in wait.
At the now-destroyed Arcturus Station, Admiral Singh of the Third Fleet had already pulled his command ship, the SSV Logan, back to an ideal firing position for its mass accelerator cannons. The dreadnought’s guns managed to slow down a destroyer before it could demolish the Third Fleet, but Singh was forced to retreat in the face of overwhelming opposition from the Reapers.
The First Fleet, meanwhile, lost the battle at Charon hopelessly. By the time Admiral Hackett issued the order to retreat, the fleet - once the largest in the Alliance navy - had been cut in half. Commanding Admiral Ines Lindholm made the painful decision to use a tenth of the fleet’s remaining vessels as cover so the remainder could escape. Her gamble paid off, as the First Fleet limped out of the relay to rally with the rest of the Alliance forces on the run.
The Fourth Fleet, near Earth, had a few minutes of advance warning, but would ultimately fare no better than the First.
After destroying Earth’s comm buoys, smaller Reaper destroyers wiped out all GPS and communications satellites in orbit, and cut the undersea fiber-optic cables that linked the continents. Earth’s resistance now relied on outdated radio towers and a few quantum entanglement communicators whose matched pairs happened to be on other continents or outside the Sol System. Communication was so limited that the fate of entire nations remained unknown.
The capital ships bombarded defense installations and industrial centres, annihilating entire cities with populations in the low millions, including Adelaide, Hamburg, Al Jubail, and Fort Worth. Meanwhile, Reaper destroyers descended into the atmosphere to melt roads and capture larger population centres with minimal loss of life.
Shepard and T'Soni on Mars
Admiral Hackett ordered the Normandy to the neighbouring world of Mars to locate new Prothean data that could help defeat the Reapers, but the archives on Mars were not responding.
Having previously recovered encryption keys from a Prothean Shrine on Kahje, Dr. Liara T'Soni had traveled to the Mars Archive to see what she could uncover. While examining the archives, she found schematics for an anti-Reaper "superweapon", the Crucible, and recognized their significance. It's unknown who initially began the development of the Crucible, but countless species obtained and made contributions to the design over the course of millions of years, none successfully deploying it before being wiped out. As the latest species to try, the Protheans had been able to construct the Crucible, but before they could deploy it, infighting broke out between those who wanted to use it to destroy the Reapers and a faction that believed they could use it to control them; these separatists were later discovered to be indoctrinated. Despite the Crucible's elegant design, modern scientists could only determine that the device exploited the technology of mass relays, and were left to speculate on how it would ultimately function.
Shepard and T'Soni turned the schematics over to the Alliance, who brought them before the Citadel Council. Despite human Councillor Udina’s insistence that Earth was bearing the brunt of the attack and needed the most help, the asari Councillor succinctly summed up the majority viewpoint: while the Reapers focused on Earth, the other races had time to regroup and build up their own defences. The Council was also unimpressed by the Crucible schematics, insisting that the weapon was surely flawed, given that it had failed to save the Protheans themselves from extinction. T’Soni disagreed, explaining that a missing component referred to as “the Catalyst” prevented its completion during the Prothean cycle.
Meanwhile, the Alliance military abandoned the Hades Gamma cluster to incoming Reaper forces.
The Systems Alliance in trouble
Overlooked in the initial assault on Earth, the worlds of the Exodus Cluster now attracted Reaper attention. On Ur, the habdome that sheltered the colonists was destroyed when a patrol frigate in the Seventh Fleet was shot down by a Reaper Troop Transport, and two Troop Transports and a Processor landed to deal with the colonists. Unknown to the colonists, Ur was riddled with indoctrinated Reaper assets, both in its tiny local garrison and in the civilian population. Thousands died or were processed in the first few days, and the fight on Ur quickly turned into a terrified, doomed resistance movement.
The Reapers landed on Terra Nova after a brief battle with the Alliance's Sixth Fleet. Aware that it would lose, the fleet retreated in hopes of meeting with allies and retaking the planet. Intelligence indicated that the Reaper force here was light, and debate raged over whether to try to liberate the planet, or if such an effort would prove futile if the Reapers sent reinforcements.
There were a few unqualified successes. Alliance intel reported Reaper forces headed toward Uqbar, but the captain of the cruiser Shanghai quickly dispatched all its shuttles to help transport hundreds of colonists onto the ship. Making over 41 shuttle runs in a single hour, the Shanghai managed to evacuate the entire remaining population. When Reaper forces arrived, Uqbar’s capital city was as empty as if it had never been inhabited at all.
The Alliance set up supply lines from Eden Prime to funnel excess provisions to its troops.
Two Solace-class hospital ships, the SHSV Comfort and the SHSV Consolation, were destroyed from within by indoctrinated wounded. Security aboard surviving hospital ships was tightened to the point of paranoia.
Meanwhile, out in the Traverse, the Eighth Fleet was ambushed above the world of Ontarom. Although the Fleet successfully held on to the planet and its vital communications arrays, close to half of the fleet's vessels were destroyed or damaged beyond repair and Marine units on the surface took heavy casualties. Bereft of its dreadnought and with a carrier destroyed the Fleet was restricted to hit and run attacks against Reaper forces in the Traverse.