Overwatch

Overwatch is the general self-titled name of the Combine's military forces on Earth. It is used as the suffix designation of the forces in the various sectors such as Sector 17 Overwatch for City 17 and the surrounding 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.

Overview
The Forces of the Overwatch consist mainly of Combine Soldiers supported by Synths and divided into distinct "sectors" designated around an urban center, though forces other than I AM AIDS Sector 17 Overwatch are not directly encountered in the games. They are overseen by the Overwatch Voice as their immediate commander which issues orders and instructions to fulfill its objectives on behalf of the Combine government.

Role
At the beginning of Half-Life 2, the Overwatch apparently control most of Earth, corralling most of the human survivors into the few 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 shields and outposts to restrict access for example.

The main base of power for the Overwatch are the Citadels located at the heart of each City. The Citadels house legions of soldiers as well as manufacturing and repairing support Synths like the Gunship. On the other hand, it isn't clear if the Combine Soldiers and 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. The Overwatch Voice is also known to have been housed in the Citadel as well.

Sector 17 Overwatch
Sector 17 Overwatch is the official self-titled designation of the division of the Overwatch garrisoning City 17 and the surrounding environment, including The Coast and Nova Prospekt. This division is believed to be the best of the best and the largest garrison of Overwatch forces on Earth due to the prominence of City 17 as the Combine's base of power with Breen living in the City 17 Citadel. This force is known to be massive, during the course of Half-Life 2, large troop deployments are witnessed on many occasions; gunships and Hunter-Choppers are a frequent occurrence in times of alert and thousands upon thousands of City Scanners can spill out of the Citadel on a moments notice. Overwatch Elites are also seen relatively frequently as guards and shock troops.

In spite of this, Gordon Freeman proved a continual thorn in their side since his reappearance. Dr Breen directly displays his displeasure in a private speech to the transhuman arm of the Sector 17 Overwatch that, despite Gordon's lack of military and stealth training, he had still evaded and destroyed their forces so many times.

Airwatch
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". This clearly indicates that, in the Combine hierarchy, Overwatch has the authority to dispatch Airwatch forces for air support.

Airwatch is known to include gunships but it may also include dropships and Hunter-Choppers.

Tactics
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 shield projectors, sealing doorways and securing areas so that only they can pass through them.

If the opposition continues to remain stalwart against all these strategies then heavy weapons like Striders and Gunships may be called in to literally blow the target's cover away.

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.