The allied alien Invasion of Earth convinced some U.S. military commanders that they couldn't always leave the planet's defense in the hands of the JLA. The Human Defense Corps, an official branch of the U.S. armed forces, is composed of 10,000 non-superpowered soldiers selected from the ranks of those who have been decorated during previous alien campaigns. HDC soldiers train in the orbital sattelite Fort Olympus and in the Area 53 complex on the ocean floor. They are equipped with the very best military gear, including ES-2 pulsar rifles and robotic S.A.R.G.E. reconnaisance vehicles.

On the HDC's first mission to Bulgravia, Sergeant Kelly's squad ran into demonic vampires who melted when sprayed with "holy napalm." The vampires managed to snatch 66 soldiers and carry them off to the underworld, so the HDC suited up to invade Hell and rescue their comrades. During this mission Sergeant Kelly learned that a previous encounter with a vampire had left him part demon, and he now rules a realm of Hades in the name of the U.S.

The Human Defense Corps was taken over by General Lane and Project 7734 and was used as a strike force against extraterrestrials.