Prisoner of war

A Prisoner of War (POW, PoW, or PW) is a combatant who is imprisoned by an enemy force during or immediately after an armed conflict.

Even in the 26th Century, the UNSC continues to take prisoners of war in their conflict against the Covenant, often using them as manual labor to construct fortifications for defending against Covenant attack and digging graves for Covenant and UNSC dead. After the destruction of Alpha Halo, multiple Sangheili were recovered in their lifepods by a UNSC-requisitioned prison ship, the Mona Lisa and experimented on by an Office of Naval Intelligence medical officer seeking ways to harness the Flood, either to find a "cure" or to use it as a weapon against the Covenant. The last survivor of these, "Henry", assisted the other human prisoners on the ship and was one of only two survivors, with him and Rimmer joining a UNSC Marine squad to try and escape. For much of the war, the Covenant refused to take prisoners, executing any human personnel who were captured or who surrendered. After the discovery that only human "Reclaimers" could activate the Halo Array, the Covenant kept them alive for such a purpose. . After the ascension of the Jiralhanae to the positions formerly held by the Sangheili, UNSC PoW's were routinely captured and butchered for food for the Jiralhanae

The Prisoner of War Medallion is awarded to Soldiers, Sailors, Airmen and Marines who have been recovered from, escaped, or released from capture by the enemy.

PoW's in the Halo games
Though the Covenant have repeatedly been stated to never take prisoners, the games depict UNSC captives in Covenant custody multiple times.
 * Halo Wars
 * Relic Approach - a number of Warthogs are trapped behind Covenant energy shields, guarded by infantry and Ghosts.
 * Anders' Signal - Professor Ellen Anders is captured by the 2531 Arbiter, and imprisoned within a stasis field on a discovered Forerunner Shield World, because of her ability to activate Forerunner technology.
 * Halo: Combat Evolved
 * Truth and Reconciliation - Captain Jacob Keyes and several Marines are rescued by the Master Chief and a team of Orbital Drop Shock Troopers aboard the Covenant Cruiser Truth and Reconciliation. Many more Marine bodies can be seen throughout the ship.
 * Halo 2
 * Gravemind - several Marines are rescued from prisoner cells by the Master Chief, as he rampages through High Charity, following the Prophet of Truth. They are either killed by the Covenant, or infected by the Flood when it infects the holy city.
 * Halo 3
 * Sierra 117 - Sergeant Major Avery Johnson and three other Marines are captured by Jiralhanae-led Covenant forces during the UNSC's extraction of the Master Chief, and used as bait to draw the Spartan into a trap. They are rescued, and manage to fend off Covenant counter-attack.
 * Crow's Nest - Marine prisoners are tied up in the barracks of Crows Nest. They were killed and tortured until the Master Chief manages to save them.