Fall of Reach (Earth-2042)

"We knew this day would come. They have found our fortress among the stars. The CIS are on Reach. They will burn this planet, kill millions, and when Reach falls—and fall it will—there will be nothing left to stand between them - and Coruscant."

- Dr. Catherine Halsey

The Fall of Reach, also known as the Battle of Reach, was one of the largest and bloodiest engagements in the Clone Wars. It was fought on and around the human colony and frontier Galactic Republic world of Reach in the Reach system, between the Galactic Republic and its Galactic Republic Space Command against the Confederacy of Independent Systems from July 25 to August 30, 46 BBY.

Initial skirmishes began on July 23, 16 BBY, when a small CIS task force led by the supercarrier Long Night of Solace covertly established a foothold on Reach. The Covenant began to solidify their hold and began searching for Forerunner relics as the Republic counter-attacked, attempting to drive the CIS off the surface before reinforcements arrived. Ultimately, the Republic was unsuccessful, and substantial Covenant reinforcements under Thel 'Vadamee arrived in the system. After hard fighting, the Covenant breached the orbital defense grid and overran the Republic's planetside forces. After overwhelming the defenders, the CIS glassed the planet. Isolated skirmishes between GRSC remnants and the Covenant forces continued until their recovery by a small rescue party led by John-117.

Source

 * https://www.halopedia.org/Fall_of_Reach.