Who are the HECU? "The Hazardous Environment Combat Unit, or HECU for short, is a United States Marine Corps Special Forces unit that was deployed to Black Mesa following the Resonance Cascade disaster. Most were given orders to eliminate all hostile Xen aliens that had teleported into the facility, as well as eliminating science team personnel as part of a government cover up. They were disbanded along with the US Government following Combine Occupation, though some may have survived."
The Lore master is here!
Adding to this - it has been said that the Hazardous Environment Combat Unit has its troops from: Marines, Army and Navy. The bulk of the HECU is made up of marines though.
The bulk of the "boots-on-the-ground" HECU forces are Marines. The HECU falls under MARSOC (Marine Forces Special Operations Command), a branch of the main USSOCOM (United States Special Operations Command). SOCOM houses special forces for Army, Marines, Navy, and Air Force and at times, will conduct joint operations. That also means sharing assets, intel, and any supplies needed for mission objectives. Thats why we see many Army Apaches and heavy armored tanks supporting the HECU.
Picture credit to TheParryGod