The Combine are a race that expand over parallel universes, taking over planets for resources. They use advanced technologies to artificially evolve both themselves and the native creatures of whichever planet they have enslaved. Because of their dependence on technology the real Combine have become large slug-like creatures known as Advisors. Despite their outward appearance, they are very powerful and possess strong telekinetic abilities, being able to lift people and objects into the air as well as having the power of flight. Even though the Combine can teleport between universes, they cannot teleport to different places in the same universe. The Combine forces are the main enemies in Half-Life 2 and its two episodic expansions.
These enemies are either human or are modified humans.
These are unmodified humans who patrol the streets keeping people in line through intimidation and thuggery. They always carry an electrical stunstick and will not hesitate to beat any citizen who steps out of line. Some carry a 9mm pistol or a SMG. They often carry deployable manhacks to help flush out rebels. People join the CP so they get benefits like better living conditions and more food. Resistance members sometimes manage to infiltrate the ranks of the Civil Protection, such as Barney Calhoun. They can be identified by their distinct white mask and robotic voice.
The Overwatch makes the backbone of the Combine's forces on Earth. They are extensively modified humans - they have brain and chest surgery, throat modifications, as well as various mechanical devices implanted into the skin. The brain surgery removes some emotion and memory - however soldiers still exhibit some human qualities such as pain and fear of punishment. This procedure happens at both Nova Prospekt and the Citadel. They usually carry SMGs and Pulse Rifles. Some carry a shotgun - they are more powerful and have the Elite red band on their arm. They always carry a grenade and will throw it at anyone hiding behind cover. They travel in dropships and APCs.
The Elite soldiers are even more heavily modified than Overwatch soldiers - they have more armor and their mask has one large central red eye - it is unknown what is underneath an Elite soldier's mask - the eye may just be for show. These forces are more highly trained than the Overwatch and CP Combine and always carry a Pulse Rifle.
The product of extreme and brutal Combine engineering, a Stalker is a human being who has had his or her body physically altered by the combine via savage medical means. Their limbs have been removed along with most other organs, fat, muscle tissue and reproductive gametes, instead being augmented with what appears to be metallic prosthesis, grafted to parts of their body. Stalker's are not known to be possessed of speech - whether or not they are even still capable of such-, most likely due to the removal of key vocal structures such as the vocal chords or perhaps entire larynx itself as the metallic hole in their throats suggests. For ease of maintenance and to bring a physical dependence on the Combine, the Stalker's digestive tract has been completely removed, resulting in the Stalkers surviving solely on a saline solution that the Combine provides. Furthering their degeneration into a vestigial organism. All Stalkers also appear to be genderless as a result of their heavy re-engineering at the hands of the Combine empire. They perform the most basic functions for the combine empire, interacting with combine technology via a laser that can be deadly if trained on humans.
Synths are the result of the Combine's experiments on other planets. They are both organic and machine and may be far from what the original creature looked like.
The Strider is a colossal beast that is very dangerous and immune to most weapons. It has a pulse gun and a cannon that fires a deadly laser which will literally disintegrate most objects. They can be destroyed by rockets, energy orbs and Magnusson devices.
These creatures fly around transporting every kind of combine force - usually Overwatch soldiers, but they can even take huge striders and APCs. They also deploy rollermines on the coast.
Gunships are powerful aircraft armed with a pulse cannon. They are often deployed to places ground forces can't get to, or important locations. They have the ability to shoot down rockets but can be fooled by quick movements in different directions.
These pods are used to transport Advisors from the Citadel, so they are heavily armored and can fly or glide long distances.
These fast units are used as scouts and escorts to Striders. They are smart and will follow rebels into houses, smashing through walls or windows to get in. Hunters possess a powerful flechette launcher and will also ram people who get too close. They are quite well armored but have a weakness to objects thrown with the Zero Point Energy Field Manipulator.
These synths are seen inside the Citadel on conveyor belts, but have not been seen in combat.
The Combine have modified certain human technology for their own purposes.
The APC is dangerous by itself, having a rocket launcher and a pulse rifle, but it also carries troops to deploy them quickly to an area. They are used a lot in the canals to try and intercept Gordon Freeman. They can also be used to temporarily power things like forcefields, which is useful to hastily set up a base.
The Combine's version of a helicopter, the Hunter-Chopper is armed with missiles, mines and pulse guns. They are usually seen closer to City 17 and in the canals - on the coast they are mostly replaced by Gunships.
These flying devices are deployed around City 17 and the canals. They fly around to their target, chopping them up with the spinning blades attached to them. Some Civil Protection carry them.
Scanners are used to search for people and keep an eye on the population. They have a camera with a blinding flash. They have no weapons but when damaged they will attempt to crash into its attacker.
The Combine are not mentioned during the original Half-Life but it is now known The Nihilanth escaped to Xen to hide from the Combine who were attacking its homeworld.
Between Half-Life and Half-Life 2
The Combine came to Earth through portal storms that allow travel between dimensions. They also learned to use Xen to teleport - as a 'dimensional slingshot'. They launched a huge invasion known as the Seven Hour War (because the humans surrendered after seven hours). Dr. Wallace Breen negotiated the surrender and was appointed the Administrator of the Combine forces on Earth. The Combine then proceeded to create large cities with huge towers known as Citadel's, one being City 17 in Eastern Europe. The cities have severe restrictions, but due to the danger outside of the cities from Antlions and other creatures, most humans opted to stay inside the cities. The Combine set up a Suppression Field that stops embryos forming, stopping reproduction. Memory loss due to poisoned drinking water is used to stop people rebelling and those that do rebel are killed, their bases destroyed by headcrab shelling and combine footsoldiers. The combine also commit acts of genocide, such as the one seen at Ravenholm where the entire population bar one are headcrab zombies. They began to take Earth's resources, going so far as to drain the oceans.
By the time Gordon Freeman arrives, the Combine have completely enslaved the populace and have instilled fear into them. Some people are being used to make stalkers and other creatures. The entirety of City 17 and probably the world has a Big Brother feel to it - the population is watched and is pumped with Combine propaganda courtesy of Breencasts - a television program that has Dr. Breen make long speeches about how the Combine rule is for the greater good and is the next step in the evolution of the species. These programs are on small televisions but they are also shown on massive screens around the city, meaning they are always seen. Once the Combine discover Freeman they go to great lengths to try to stop him from causing any damage, to no avail, as Gordon destroys many Combine forces. The large prison and Combine base Nova Prospekt is destroyed when the teleporter explodes. This sparks a huge rebellion in almost the entire population. Fighting is fierce for over a week and Gordon is thought dead until he arrives back in City 17 because of a 'slow teleport'. Gordon proceeds to the Citadel and wreaks havoc, making it all the way to Breen's office at the top. The Citadel is severely damaged.
Half-Life 2: Episode 1 & 2
The Combine forces are in disarray and few remain defending the Citadel. They, along with the humans attempt to escape from the city. The Advisors, who have been hiding in the Citadel away from the humans, are seen for the first time being transported in pods, eventually being ejected from the Citadel before its destruction. The Citadel explodes and the Combine attempt to create a superportal to call in reinforcements - they are completely cut off from their homeworld and any other Combine colonies. The data packet Alyx Vance stole from the Citadel contains a way to neutralize the superportal, so the Combine attack them and the White Forest base hoping to destroy the rocket being used to send the transmission. The Combine, however, are defeated and the superportal is neutralized, leaving the Combine stranded. But two Advisors manage to kill the rebel leader Eli Vance, and possibly even extract information from his brain in the process.