From Halopedia, the Halo wiki
Following their first meeting and ensuring battle with the San'Shyuum at Ulgethon, the Sangheili could not best their enemy's powerful Dreadnought, an ancient Forerunner keyship that the San'Shyuum had brought from their homeworld. Eventually, after losing several of their colonies, the Sangheili came up with a plan to defeat the vessel, deciding to sacrifice Codisfold in a daring attack. To prepare, they hid dozens of ship-boring machines in the rings around the planet. When the Dreadnought arrived to destroy Codisfold, the ship-boring machines launched from their hiding places, carrying skilled infiltration teams toward the keyship. The Sangheili managed to sneak aboard the Dreadnought, damaging several of the vessel's critical systems and slaying thousands of San'Shyuum. While taken by surprise, the San'Shyuum counterattacked and killed all of the Sangheili intruders. Following the incursion, the San'Shyuum attacked Codisfold, killing millions of Sangheili residents.
While the raid on the Dreadnought was ultimately a costly failure for the Sangheili, it forced the San'Shyuum to acknowledge their vulnerability. The unrest it caused among them would help pave the way toward a truce between the two species, and ultimately the formation of the Covenant.