This article contains spoilers. Read at your own risk!
It might spoil your gaming experience when first playing the related games.
The Combine Overwatch, also known simply as Overwatch, is the general self-titled name of the Combine's military and police forces on Earth. It is used as the suffix designation of the military forces in the various sectors such as Sector 17 Overwatch for City 17, Quarantine Zone, Nova Prospekt, and other parts of the world, as well as the wasteland. The name also explicitly refers to the Combine's automated Command and Control System on Earth, often encountered in the form of the Overwatch Voice. Although primarily made for garrisoning Earth, Overwatch soldiers are hinted to be deployed throughout the Universal Union on various off-world assignments in other territories.
The Combine Overwatch primarily consist mainly of a wide variety of unit types, such as the common Combine Soldiers, supported by a range of many types of Synths forces as support. Overwatch forces are divided into distinct "sectors" designated to them depending on their general location, such as Sector 17 Overwatch as they focus around an urban center, though forces other than Sector 17 Overwatch are not directly encountered in the games.
Day-to-day military activities and operations are overseen by the Overwatch Voice, an artificial intelligence that acts as Overwatch Command. The Overwatch Voice issues orders, instructions as well relaying as information to the Overwatch forces to assist and fullfill their objectives. They also have a command structure, from squad leaders leading whole squads, to an entire Sector being led by a sector leader. As seen at Nova Prospekt, a high ranking Combine officer inquires the status of several squads along the coast on the radio, and even field promotes one of them to command the rest, when the previous one fell to Gordon Freeman's hands. During the events of Half-Life: Alyx, Overwatch Command were inquiring damage assessments from a Combine grunt stationed on the ground beneath The Vault, before needing to send reinforcements into the Quarantine Zone.
The Citadel located in City 17, acts as main base of operations for the Combine Overwatch on Earth, with multiple bases, garrisons and other strongholds throughout the many cities and other territories. The Combine forces on Earth, are under the administration of Dr. Breen, who serves as Earth's Administrator, with the Advisors, a race of aliens being responsible for overseeing all activities and communications between Earth and the rest of the Universal Union.
In the aftermath of the Seven-hour war, the Combine, having defeated all of Earth's Military, established their presence on Earth. Dr. Breen, former administrator of Black Mesa, was appointed Humanity's representative, and Earth's Administrator. In order to enforce law and order on the newly occupied world, the transhuman arm of the Combine Overwatch was established in order to fulfill the role as the enforcer on the behalf of the Combine on Earth.
At the beginning of Half-Life 2, the Overwatch apparently control most of the Earth, corralling most of the human survivors into the remaining cities where they are strictly controlled, watched and put to work for the Combine's own ends. Most of the day to day details of this work is overseen by Civil Protection with the Overwatch forces usually only being called in to deal with heavy resistance whilst manning outposts and checkpoints beyond the city limits. The road along the coast to Nova Prospekt is heavily patrolled using aerial dropships, gunships, force fields and outposts to restrict access for example.
The main base of power for the Overwatch is the Citadel located at the heart of the City. The Citadel house legions of soldiers as well as manufacturing and repairing support Synths like the Gunship. The Overwatch Voice is also known to have been housed within the Citadel. On the other hand, it isn't clear if Stalkers themselves are 'upgraded' from the human population inside the Citadel itself as that task seemed mainly to be performed in Nova Prospekt during Half-Life 2.
Sector 17 Overwatch
The Sector 17 Overwatch is the Combine's strong arm garrisoned within City 17 and its outlining regions including The Coast and Nova Prospekt. As being headquarters for Dr. Breen and the Combine, the Sector 17 Overwatch contains the most troops and synths, sporting a colorful mix of human, transhuman and Synth units for deployment against the resistance.
The Metropolice forces of the Sector 17 Overwatch are the official police organization stationed in City 17. Their jobs include policing the city for anti-civil activity, overseeing work operations, and generally keeping the citizens in a permanent state of fear of their overlords. They are permitted and almost seemingly encouraged to utilize severe beatings as a means of coercing the population into submission, initiating raids of individual apartments to search for contraband material at random intervals, and summarily beating citizens for malcompliance. They are the face of the Overwatch that the player will encounter strongly for the first four chapters of the game, after which their role becoming absorbed by the other branches of the Overwatch.
This branch is the primary division utilized by the Combine as the general military ground force. "Transhumans" are people who have been surgically modified on the brain, neck and general body with the Combine technology, which leads to full obedience, emotional suppression and enhanced fighting skills. Trained in unit tactics and armed to the teeth, the transhuman forces are often represented by the basic Overwatch Soldier unit that the player encounters throughout the game. Variety does exist to a degree with the introduction of the Overwatch Elite soldiers, transhuman units deployed either as squad leaders over regular soldiers, or in squads of their own as shock troopers. A small number have been garrisoned at the Nova Prospekt prison and the Depot, sporting different colored uniforms but similar capabilities. This branch sports a much larger variety of equipment than Metropolice with distinct firearms utilizing pulse technology over typical human designs. The Combine Grunt, Combine Suppressor, Combine Ordinal and Combine Charger that are seen in Half-Life: Alyx, are not seen again by the events of Half-Life 2 and its episodes. These units were probably no longer needed and got replaced by the Overwatch Soldiers and Elites. A reason for that could be the failure to contain The G-Man in The Vault. The Combine probably modified the following units even more to prevent similar events from happening again. These soldiers are modified humans too but still have a bit of personality left, especially the Combine Grunts.
As the most recently-added branch of the Combine Overwatch, the transhuman branch's operations are under constant evaluation by the Combine itself. Dr. Wallace Breen is heard at one point berating the soldiers stationed at Nova Prospekt for having failed to capture Freeman, angrily reminding them that in order to become an indispensable part of the Combine Overwatch, they must "earn the privilege".
The Synth arm of the Combine Overwatch is composed of Synth such as Striders, Dropships and Gunships. Synths are bio-mechanical war machines, apparently derived from alien species previously conquered by the Combine and augmented with Combine technology. With a variety of Synth creatures at its disposal, this arm is extremely versatile and capable of tackling a variety of different operations.
During the course of Half-Life 2, the Overwatch Voice makes a reference to something called Airwatch during Gordon's attack on the outside perimeter of Nova Prospekt. Shortly afterwards, a pair of gunships attack. It is clear that Airwatch is a subdivision of Overwatch from the following communication: "Overwatch acknowledges critical exogen breach. Airwatch augmentation force dispatched and inbound. Hold for reinforcement".
When the Overwatch forces are deployed against a target near one of their power bases like the Citadel or Nova Prospekt, they prefer to employ blitz tactics to take their opponents by surprise and overpower them. Scanners locate potential targets and Manhacks distract any forces located in narrow spaces underground or indoors. However, if the opposition is well fortified and alert to any incoming attack then they will call in artillery strikes using Headcrab Shells to spread panic and confusion before pushing forwards with ground troops, attempting to flank resistance fighters as they fall back.
In indoor urban environments, a favored tactic is to destroy the doors to a room using the Combine door lock's self-destruct function to create a smokescreen of flying wood and splinters then catch their opponents in a crossfire from all sides before they regain their senses. Soldiers may also set up heavy equipment like force field projectors, sealing doorways and securing areas so that only they can pass through them.
In less accessible locations such as the coast or White Forest, ground forces are usually deployed using Dropships with support from Hunter-Choppers or Gunships to suppress resistance while reinforcements land. Such locations are usually not in range of Headcrab Shell Launchers so battles tend to be more evenly matched if the resistance forces are well armed. Interestingly, the Combine don't seem to like deploying Striders in this way despite being able to carry them on Dropships, though the terrain of the coast may simply be unsuitable or too far from the Citadel for them to be deployed in time.
- There is a small chance that when a grenade is thrown at an Overwatch Soldier they will scream "AH!" just before the grenade explodes.
- Civil Protection is the only Overwatch unit to not punch with their weapons unless they are armed with Stun Baton.