In the expansion pack Half-Life: Opposing Force they appear as allies (due to the player taking the role of an HECU soldier Adrian Shephard).
The HECU is a special operations division of the United States Marines, consisting of specialy picked soldiers who are trained and readied to operate in harsh environments that include things such as:
- Extreme Heat
- Radio-acitve Materials
- Dangerous Machinery
- Structural Collapse
- and Treacherously Manuevered Terrain
Thereby making them better qualified than regular soldiers for hazardous situations, however this usually only amounts to a couple baby-sitting jobs. HECU personel consists of squad leaders, grunts, medics, engineers, and snipers, dressed in black and white camoflauge fatigues, and a Powered Combat Vest, with several miscellaneous items such as ammo pouch belts, gas masks, balaclavas, and night vision goggles. HECU soldiers tend to be heavy-handed, being eager to kill anything that moves and going out of their way to kill it, often getting upset because it's not challenging enough.
The HECUs biggest (and last) mission was to take care of the Black Mesa incident, a large interdimensional rift leaking hostile aliens throughout the Black Mesa Research Facility in New Mexico. However the HECU were not told what their mission was when they headed out, it was believed to be nothing more than another baby-sitting job by soldiers, but as the soldiers entered the uncontrollable chaos that was the Black Mesa incident they knew it was anything but. It was there that the soldiers got their orders to cover-up the situation, meaning kill all the aliens, and the scientists, and especially, Gordon Freeman. The HECU entered armed with helicopters, tanks, artillary, and more than enough ammo to take on the alien forces as best they could, often remaining in a stalemate between the two that would eventually be broken by Freeman.
Gordon Freeman was working as a research associate at the Black Mesa Research Facility, where he was working in the anti-mass spectrometer in the Anamolous Research labrotories of Sector C. It was there that the spectrometer failed disastrously and was destroyed, thereby opening the interdimensional rift between Earth and Xen, officially marking the start of the Black Mesa incident. With this knowledge in hand the HECU were convinced that Gordon had sabotaged the spectrometer and caused the rift purposely, making him their main target in the operation. The HECU were therby reluctant in stopping Gordon from escaping the facility, using whatever means they could to stop him including, tanks, helicopters, and heavy explosives. But with all of this combined in their ultimate anti-Gordon effort he still managed to prevail, taking down everything and everyone in the way of his goal to stop the invasion. The HECU finally decide to forget about Freeman and cut their losses by retreating from the facility and launching airstrikes throughout the facility, leaving behind only a number of "last chance" soldiers that are still determined to take him down with them.
- "This place makes boot camp seem like Disney Land."
- "This mission is way beyond fubar."
- "I don't care what we're goin in for, as long as I get to kill me somethin."
- "You've gotta get out of here..listen to me Shephard..those things..they'll kill all of us."
- "Don't..go in there corporal..that monster..whiped out my..squad. Aaaargh."
- "I killed twelve dumbass scientists and not one of em fought back. This sucks."