A Borg attack in 2367 that destroyed 39 of the 40 Starfleet ships sent to stop it and killed about eleven thousand people.
The Battle of Wolf 359 was fought in 2367, when a single Borg cube invaded Federation space and headed for Earth. The Borg had captured Captain Picard and assimilated him as Locutus, giving them all of his knowledge of Starfleet.
Starfleet gathered a fleet of forty ships under Admiral J.P. Hanson at the Wolf 359 system to stop the cube. Because the Borg knew Starfleet's tactics through Locutus, the battle was a slaughter. Thirty-nine ships were destroyed, and about eleven thousand people were killed. The Enterprise-D, under Riker's command after repairs, arrived only after the fighting had ended and found a field of wreckage.
Among the survivors was Lieutenant Commander Benjamin Sisko of the USS Saratoga, whose wife Jennifer was killed, and he long blamed Picard for her death. The Enterprise crew later rescued Picard, and Data used his link to the Collective to send a sleep command to the drones, which made the cube destroy itself near Earth.