The Reapers took Earth in a matter of hours. The Alliance knew the first wave would arrive from batarian space, but they were unprepared for the speed and scale of the attack.
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 but was soon destroyed. The Fourth Fleet, near Earth, had a few minutes of advance warning. It stood no better chance.
After destroying Earth's comm buoys, smaller Reaper destroyers wiped out all GPS and communications satellites in Earth's orbit and cut the undersea fiber-optic cables that linked the continents. Earth's resistance now relies on outdated radio towers and a few quantum entanglement communicators whose matched pairs happen to be on other continents or outside the Sol system. Communication is so limited that the fate of entire nations remains unknown.
The capital ships bombarded defense installations and industrial centers, 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 population centers with minimal loss of life. This is not an example of the Reapers being merciful. More likely, they are herding their prey to make the coming harvest that much easier.