Prisoner of war

A prisoner of war (POW) is someone, either a civilian or combatant, who is held in custody by an enemy power during or immediately after an armed conflict.

San Hieronymo personnel
During FOX's occupation of the San Hieronymo Peninsula, several Soviet personnel on the missile base were imprisoned for a variety of reasons, including resisting authority, homesickness, low morale, and loyalty issues. Roy Campbell was taken prisoner as the sole survivor of the Green Beret unit sent to investigate the peninsula. Nikolai Stepanovich Sokolov also ended up being taken prisoner after leaking intel on Metal Gear to Naked Snake, although Snake's resistance later managed to locate and rescue him.

Peace Sentinels
The Peace Sentinels kept various prisoners of war in their bases, the majority of whom were Sandinistas, although a few had no prior ties to the FSLN, at least one of whom was also a former American soldier. In addition, some of the scientists were also kept as POWs due to inherent disagreements with their superiors within the Peace Sentinels. In addition, the Soviet soldiers also held at least one American soldier captive when they took over the U.S. missile base in Nicaragua, also wounding him.

Militaires Sans Frontieres/Outer Heaven
While recruiting potential allies during the Peace Walker Incident, the Militaires Sans Frontieres would imprison those soldiers who were reluctant to join immediately, in the brig on Mother Base. KGB spy Vladimir Zadornov was also taken prisoner by MSF after his involvement in the Peace Sentinels' plans was exposed, as well as his attempt to kill their commander Big Boss.

In 1995, Outer Heaven began taking prisoners, some of whom were members of the local resistance, including Jennifer's brother. He, along with two other prisoners, were later used as human shields by mercenary Dirty Duck during his fight with Solid Snake. Solid Snake himself also ended up deliberately becoming a prisoner of war in order to locate Gray Fox, who himself was detained in Outer Heaven after being caught in Operation Intrude N312.

FREMILO
Gray Fox was taken prisoner and tortured by the FREMILO group during the Mozambican Civil War, until Big Boss broke him free.

Iraq
Liquid Snake was taken prisoner by the Iraqis during the First Gulf War, and remained as such until he was rescued by U.S. forces in 1994.

Middle Eastern militia
Middle Eastern militiamen fighting against PMC forces in 2014, sometimes captured enemy Praying Mantis soldiers. They would then be taken to a safe house, and in some cases executed.

Pieuvre Armement
Some South American rebels were captured by Pieuvre Armement in 2014. Some were captured at the Cove Valley Village, where they awaited execution, while others were detained in a confinement facility near the Vista Mansion.

Unconfirmed usage
McDonell Benedict Miller, then known as Kazuhira Miller, was captured by the Vietnamese during the Vietnam War and interrogated and put as a prisoner of war, before eventually escaping and decapitating the head interrogator.

Behind the scenes
Prisoners of war often feature as characters in the Metal Gear series, with the player having the option to save, and in some cases, recruit them.

In the original Metal Gear, the player has to rescue a large percentage of the prisoners to gain Jennifer's trust and allow her to answer Snake's calls. In addition to thanking Snake, some of the prisoners will also supply hints to the player as to how to proceed next. Saving the prisoners also results in the player's rank increasing, which also allows for increased health and ammunition counts.

The next Metal Gear game to utilize prisoners of war was the non-canonical sequel Snake's Revenge. Like in the first game, rescuing them will allow the prisoners to offer hints on how to get through areas, as well as rank increases, although unlike the first game, they aren't completely necessary to complete the game outside of rank increases.

The concept of prisoners of war as a gameplay element was not canonically used again until Metal Gear Solid: Portable Ops, where the player can locate prisoners and rescue them, recruiting them in the process, after the Spy Unit supplies a spy report relating to their imprisonment. In the expansion pack, Metal Gear Solid: Portable Ops Plus, the maps of the Infinity Mission mode sometimes includes prisoners of war to rescue. The locations of the prisoners are indicated by an icon resembling cage bars.

In Metal Gear Solid 4: Guns of the Patriots, Solid Snake could rescue prisoners of war in the Solid Sun chapter at the Cove Valley Village and the Confinement Facility, the former event results in the soldiers repaying the effort by unlocking several sheds for the player to stockpile, and the latter will result in two additional soldiers accompanying the player when (s)he arrives at the Vista Mansion. Both instances also result in increased affinity in regards to the soldiers.

In Metal Gear Solid: Peace Walker, the player can fulton several prisoners of war and recruit them into MSF. The prisoners are identifiable as characters in yellow jumpsuits with the letters "P.S." on the back. Upon being recruited, they are identified as having the symbol "POW" near them with a gold background. Several Extra Ops missions also have rescuing prisoners as being a required element of successfully completing the mission.