The Keyes Loop was a famous space tactical maneuver first executed by the then Commander Jacob Keyes at the Battle of Sigma Octanus IV. The result was one of only three decisive space victories achieved by the UNSC over Covenant forces and it led to Keyes' promotion to Captain.
Summary
Arriving over Sigma Octanus IV in response to a possible impending Covenant attack, the UNSC Iroquois found itself outmatched by the arrival of a Covenant battle group consisting of one carrier, one destroyer, and two frigates (medium-sized ships). This was contrary to the expectation of four medium-sized ships.
Commander Keyes resolved to attack by deciding to set the Iroquois on a collision course for the destroyer at emergency acceleration while launching a nuclear warhead in what seemed the wrong direction. The two escorting frigates proceeded to launch one Plasma Torpedo each, but the Iroquois managed to evade by utilizing the emergency thruster. Having missed their target, the plasma came about and continued to track and home in on the UNSC warship.
The trailing ordnance was to prove critical to the success of Keyes' strategy. A last minute course correction allowed the Iroquois to merely "graze" (scrape) the destroyer's shields rather than smash directly into the ship properly, resulting in the destruction of a large portion of the Iroquois' armor (Keyes' tore through 2 meters of titanium armor, and subsequently breached every bottom deck of the Iroquois). However, the tracking Plasma torpedoes were unable to correct themselves for the sudden maneuver of their target and, in consequence, impacted on the Covenant destroyer instead. A heavy salvo of Archer missiles sent the remaining Covenant warship to its destruction.
With the momentum built up during the first phase, the Iroquois performed a slingshot maneuver around the planet. Once safely away, the nuclear warhead deployed earlier, was detonated, completely draining the shields on the two frigates. Having completed its orbit, the Iroquois resumed its attack and destroyed the two frigates with two heavy MAC rounds and hundreds of Archer missiles left. Its escorts destroyed, the carrier then disengaged and escaped out system but not before leaving 34 dropships and a small ship, a Stealth Corvette, to continue the ground assault. Later on the Carrier hailed the rest of the attack fleet to Sigma Octanus IV.
In the after-math of the battle, the Iroquois was damaged to the point that it almost required a complete overhaul. 15 percent of the electronics had to be replaced due to EMP burnout from the Shiva. Also, 8 magnetic coils had to be replaced, disabling the MAC for a time. One of the emergency thrusters had to be refilled. The engines vents had fused together due to extreme heat and would need a complete overhaul to get them fully replaced. Even after refit aboard the Cradle, only 50 percent power was available. Finally, all of the bottom armor and bottom decks had been eroded by the Covenant destroyer's shields.