Balaclava

A balaclava is a full head mask usually composed of warming materials like cotton or heavy cloth. It typically covers the entire head down to the base of the neck, often with two openings in the front for the wearers eyes. Balaclavas make many appearances in the Metal Gear universe, usually being worn by enemy soldiers, though there are exceptions to this rule. A more common name for balaclava is "ski-mask."

Origins
Balaclava masks are named after the town that created them, specifically Balaklava in Ukraine. They were originally made in large numbers during the Crimean War to protect British soldiers from the nations bitter cold. The garb quickly grew popular among the british soldiers and eventually gained world wide appeal, especially in colder nations like Russia. Many military forces who specialized in winter combat made use of them and continue to do so to this day.

Virtuous Mission/Operation Snake Eater
Naked Snake had many encounters with GRU and Spetsnaz. Ocelot soldiers who wore modified balaclavas that were made of a thinner, leather-like material then the winter designed versions. These balaclava's were not designed to retain heat; the high temperature and humidity of the jungles and high plateaus of Tselinoyarsk made such a feature unnecessary. Instead, the masks were meant to ward off minor hazards such as insect bites and stings to the wearers face. The masks also protected the wearer from outstretched branches and thorn covered bushes, making traversing through the dense jungle foliage easier. The balaclava's used by the GRU soldiers differed from those worn by Ocelot operatives in that they were gray in color, with the Ocelot masks being black. The Balaclava also mitigates the tear-gas effect caused by the detonation of smoke grenades. The Pain, a member of the Cobra Unit, also wore a balaclava to disguise his deformed face, removing it only to start firing bullet bees from his mouth.

One GRU soldier in particular, Johnny Sasaki (Sr.) wore a special balaclava with the letter "J" emblazened on the forehead. He was the only soldier to have a mask with such a feature.

San Hieronymo Takeover
The San Hieronymo Takeover was orchestrated with Russian soldiers who recieved the same basic training as the GRU forces Naked Snake encountered during the Virtuous Mission and Operation Snake Eater, so it is fitting then that these soldiers also came with identical weaponry and attire, including the trademark GRU Balaclavas. While the masks served little practical use in the dry, urban environments of the Heironymo missle base, they were still worn as a matter of unit discipline and conformity.

For Naked Snake's personal uprising however, the masks proved an invaluable asset. With all the Soviet soldiers appearing virtually identical thanks to their face obscuring masks, Snake was able to send soldiers cooerced to his side through many key locations with next to no chance of detection, save by the more observant eyes of soldiers and personnel not dressed in the same uniform.

Peace Walker Incident
Big Boss's military company, Militaires Sans Frontieres (MSF), made use of a wide range of garb and armor, sometimes for matters of camouflage, sometimes for matters of protection, and even occassional matters of formality. But one article of clothing that remained constant throughout all these battle dresses (minus the odd tuxedo or bathing suit) was the standard issue MSF balaclava. Composed of seven high density textile sections and lined with well ventilating foam insulation, the MSF head mask was one of the most advanced articles of war clothing during its time, granting the wearer superb protection from even the thickest debris kicked up in heavy battle. While the material did not provide any real defense against bullets, it did greatly cushion the more sensitive parts of the skull (the back of the head, the bridge of the nose, and center of the jaw, and the temple regions) from serious injury during close quarters combat, allowing a trained soldier to take a pummelling and still retain conciousness. The masks also came equipped with two codec earpieces and a miniaturized microphone embedded in the mouth section, granting two way communication with MSF Mother Base to coordinate mission objectives. The MSF balaclava was even worn during training exercises, and was considered a standard part of the MSF uniform. Only Big Boss himself was permitted not to wear one during missions, which he did out of personal preference.

Hot Coldman's forces also wore balaclavas to protect themselves from jungle nuisences like insects or foliage. Helmets were often worn in combination with the masks. The American faction of the Peace Sentinels wore visor-type balaclavas, whereas the Soviet faction of the Peace Sentinels wore two-eyed balaclavas similar to the Militaires Sans Fronteires.

Shadow Moses
During Liquid Snake's takeover of the Shadow Moses nuclear disposal facility, his personal military, the genetically modified Genome Army, was forced to endure the extremely inhospitable conditions Shadow Moses Island offered. While each Genome Soldier was heavilly modified to be stronger and more durable the the standard human being, not even they could endure the sub-zero temperatures of the frozen arctic without proper attire. Thus, in addition to thick jackets, pants, and other warming paraphanalia, each Genome was provided a winter balaclava to keep their heads warm and functional. Like the rest of their winter garb, the balaclavas were made of white material to let each soldier blend in with the surrounding snow, though this was of little consequence inside the actual facility. Some soldiers were granted uniforms and balaclava's of different hues for this reason. Because of the chilly atmospheres ability to penatrate into the various buildings through the ventalation ducts, the Genome Soldiers were made to wear the masks even when not patrolling the outer perimeters, which proved very handy for the daring Meryl Silverburg, who donned a full Genome uniform in order to infilitrate the complex undetected.

The Big Shell Incident
The Gurlukovich Mercenaries, a Russian army-for-hire, had long used balaclava masks as part of their standard attire even before joining forces with Solidus Snake's Sons of Liberty terrorist group. The reasons for wearing these masks had less to do with a matter of protection or insulation, but instead served as a way to conceal identity. With the Gurlukovich forces seeing combat all across the globe, the need to hide their persons proved necessary to make themselves as difficult to trace as humanly possible; thus, they made use of balaclavas even in places where they wouldn't normally be worn, such as the inner workings of a tanker or, in the case of the Big Shell Incident, the sea swept scaffolds of the Big Shell.

The Gurlukovich balaclava was made of black, skin tight fabric, making it easier for the wearer to don additional headgear, such as helmets or night vision goggles. Being that it's primary purpose was to hide identity, the masks provided little warmth to the wearers head, though in the case of the sun drenched Big Shell, that wasn't necessarilly a bad thing.

During his infiltration of the Big Shell, the "FOXHOUND" agent Raiden temporarily donned a full Gurlukovich uniform, balaclava included, in order to make his way to the hostages held in Shell 1. It was the removal of this balaclava at the hands of Revolver Ocelot that put Raiden in high peril of being killed or captured, the intervention of Mr. X being the only thing that saved him. Similarly, Solid Snake, while disguised as a SEAL Team 10 operative, donned a balaclava.

The members of SEAL Team 10 also utilized balaclavas, presumably to conceal their identities when conducting missions.

Liquid Sun
In 2014, Liquid Ocelot's Outer Heaven PMC was one of the most powerful in the world, with it's soldiers possessing some of the best gear and equipment money could buy. Among their standard issue equipment, namely with the PMC unit Praying Mantis, were desert adapted balaclavas, woven out of micropore covered material that allowed sweat and oil to escape outward but prevented sand and dust from going inward. While being quite simple in comparison to the rest of the Outer Haven soldiers' gear, the masks were worn by all the standard infantry units under Liquid Ocelots control as a means to protect his soldiers from the harsh elements of the desert battlefield. Like those worn by the Gurlukovitch army, the masks were often worn in combination with helmets, eyepieces, and other such devices. In regards to the PMC unit Raven Sword, they wore an armored variation of balaclava, presumably for security purposes.

Johnny Sasaki Jr. also wore a balaclava of sorts that connected with the rest of his battlesuit. He often wore eyeprotection, effectively enclosing all his body, save for a stray tuft of hair, from the outside world, belying his somewhat paranoid personality. Meryl also briefly wore a balaclava before meeting Snake in the Middle East, implying that balaclavas were standard issue in Rat Patrol Team 01.

Although not Balaclavas, several of the Middle Eastern Militia members wore similar cloths over the head, known as shemagh scarves, that served a similar purpose.

The South American rebel soldiers sometimes wore balaclavas. Snake's South American rebel disguise also came with a balaclava.

Trivia

 * Every Metal Gear Solid game has included enemy forces that wear balaclavas
 * Balaclavas have been used in four Metal Gear Solid games (Original, Twin Snakes, Sons of Liberty, and Portable Ops) as a means to infiltrate enemy compounds via disguise
 * Both Johnny Sasaki's, Senior and Junior, have worn balaclavas
 * Balaclavas were originally made to protect the wearer from the cold. So far, only four metal gear solid games (Original, Twin Snakes, Snake Eater, and Guns of the Patriots) have seen them used for that purpose
 * Sons of Liberty, Portable Ops, and Peace Walker are the only Metal Gear Solid games that let the player control characters that wear balaclavas
 * Some of the Genome Army soldiers, clad in blue uniforms with blue balaclavas and blue helmets, greatly resemble the footsoldiers of Cobra, the terrorist organizaton that serves as the antagonist of the GI JOE franchise.
 * Although the Zanzibar Land soldiers were never stated nor shown to wear balaclavas, the Zanzibar Land forces were implied to wear them in Metal Gear Solid 4.