The Fall of Reach was possibly the largest battle during the Human-Covenant War, taking place in the Epsilon Eridani system, in orbit and on the ground of Reach, humanity's largest military stronghold. While Reach was defended by twenty orbital defense platforms and a fleet of over 150 warships, the planet and the entire system eventually fell to a Covenant force of over 300 ships, at the cost of approximately 700 million lives.
The attack on Reach involved two separate Covenant forces. Initially the Fleet of Valiant Prudence arrived in secret and attempted to subvert the planet's security systems and recover Forerunner artifacts. This invasion force was led by the CSO-class supercarrier Long Night of Solace, and was significantly hampered by the actions of the UNSC and Noble Team.
Noble Team's actions were unfortunately made mostly moot by the arrival of the main Covenant force, led by the Fleet of Particular Justice, consisting of approximately 300 warships. While the UNSC fleet stationed at Reach, along with the orbital defense platforms, defended the planet valiantly, the Covenant were eventually able to breach the planet's defenses by disabling the generators powering the orbital platforms ground-side.
The battle on Reach involved every arm of the military, calling to action almost every remaining Spartan-II including John-117 and Blue Team, the UNSC Navy, Army, Air Force, Marine Corp, and Noble Team. In all, the UNSC had over 350 million personnel involved in the conflict for the planet in orbit and on the surface. Very few, with the exception of the Pillar of Autumn, John-117, Blue Team, and a handful of other UNSC ships, survived.