Prisoner of war

 Prisoners of war are people, civilians or combatants, who end up captured and detained by the enemy either during or after a war is fought.

San Hieronymo personnel
Several Soviet personnel on the San Hieronymo Peninsula missile base ended up imprisoned for a variety of reasons, including resisting authority, homesickness, low morale, and loyalty issues. In addition, ex-GRU Major Ivan Raidenovitch Raikov was imprisoned shortly after his exile on the San Hieronymo Peninsula for his abuse towards his soldiers. Roy Campbell was taken prisoner as the sole survivor of the Green Beret unit sent to investigate the peninsula. Sokolov also ended up becoming a prisoner of war after his leaking details of the ICBMG to Snake were exposed, although his resistance managed to locate and rescue him.

Peace Sentinels
The Peace Sentinels kept various prisoners of war lying around in their bases, the majority of whom were FSLN members, although there were a few that 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

Militaires Sans Frontieres/Outer Heaven
During the recruitment process by the Militaires Sans Frontieres during the Peace Walker Incident, several soldiers that were captured via Fulton recovery often ended up being put in the brig because they attempted to fight the MSF upon arriving at Mother Base, often needing to be reasoned with until they join MSF. Zadornov also ended up taken prisoner after the latter was exposed for his involvement in the Peace Sentinels' plans, as well as his attempt to murder the MSF commander Big Boss.

During the Outer Heaven Uprising in 1995, Outer Heaven began taking prisoners for people who were presumably members of the Outer Heaven Resistance, one of which included Jennifer's brother who along with two other prisoners, were used as human shields by mercenary Dirty Duck. Solid Snake saved most of the prisoners.

Pieuvre Armement
Some of the rebel soldiers ended up captured by Pieuvre Armement, and detained as well as in some cases awaiting execution.

Behind the scenes
Prisoners of war were characters in the Metal Gear series that acted as a gameplay element.

In the first 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 of war will also supply hints to the player as to how to proceed next.

The concept of prisoners of war as a gameplay element was not 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, 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 Snake. 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 often 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.