Hazardous Environment Combat Unit
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The Hazardous Environment Combat Unit, also known as the HECU, was a fictional secret special forces detachment within the United States military. Although comprised of soldiers from many different parts of the Army, the bulk of the HECU is made up of 
U.S. Marines. Their equipment was of a high standard, consisting of Powered Combat Vests and durable firearms, including the M-4/M-203 assault rifle, MP5 Navy submachine gun and M-249 SAW.
Nothing is known of the formation of the HECU, but their first mission was at the Black Mesa Research Facility during the Resonance Cascade. The unit was prepped for combat in the facility months before the incident, with the G-Man playing an unknown role in the organisation. However, the specific orders of the mission were withheld from the soldiers until they were in the field. After receiving a distress call sent by Dr. Rosenberg calling for immediate support and rescue, the government sent in the HECU. However, due to the nature of the incident, the government wished to cover up the activities of the research facility. Therefore, the HECU were given orders to question the science team for information, and then kill everything in the base. Working in small squads, they began to scour the base.
After being deployed late on the first day, the HECU soon got information about a Dr. Gordon Freeman, who was at the flashpoint of the incident. Freeman soon became a priority target, to be eliminated at all cost. However, by the second day, HECU started to pull out as the opposition was mounting up: the Xen aliens were more powerful than thought and the Black Mesa personnel started fighting back. However, the evacuation was completely disorganised, an excessively large number of HECU members were left behind. These survivors now had to contend with two new foes: not only had a new group of aliens known as Race-X arrived, but Black Operations forces had moved in and opened fire on remaining HECU soldiers.
Eventually, most of the survivors were dead, killed by Black Operations, Race-X or in the final nuclear blast which destroyed the facility. The only known survivor from the soldiers left behind was Cpl. Adrian Shephard, who was detained by the G-Man after destroying the Race-X gateway.