Dulce Base. The Dulce Files, #1

The U. S. government has allowed the Grays, a race of extra-terrestrials from the Zeta Reticuli system, to establish a foothold on Earth at a place called Dulce Base, a secret underground complex near the Archuleta Mesa in New Mexico. This came about after President Eisenhower signed a treaty with them at Holleman Air Force Base in 1953, but following a surprise attack on the humans of the base in 1975, the treaty was deemed worthless and the U.
S. government was in a bind. Fast forward to 1979. A group of Special Forces - all "dead" Vietnam vets - are assembled with the sole task of taking the base back or destroying it. When ungodly horrors are discovered on the base's deepest levels, however, the mission changes from one of search and destroy to one of rescue. Aided by six super soldiers impervious to the mind attacks of the Grays, the men fight for humanity in an epic sci-fi battle you can't afford to miss.