Given the following context:  The film follows two stories, one about the civilians trying to stay alive during the disaster, having to battle both "the crazies" as well as U.S. soldiers ordered to shoot on sight. The other involves the political and military leaders trying to contain the epidemic. Set in the small town of Evans City, Pennsylvania, the central characters are firefighter David, his girlfriend, nurse Judy, and firefighter Clank. David was a Green Beret and Clank an infantryman, both having served in Vietnam. The town has seen a number of violent events, including arson at a local farm by a demented farmer. Judy and David are very concerned, since Judy is pregnant. Meanwhile, heavily armed U.S. troops in NBC suits and gas masks arrive in town, led by Major Ryder, who takes over the doctor's office where Judy works. Days earlier, an Army plane carrying an untested bioweapon crash-landed in the hills near the town, infecting the water supply with a virus code-named "Trixie," causing victims to either die or become homicidally insane. "Trixie" is highly contagious, with anyone drinking from the Evans City reservoir becoming affected. In Washington D.C., government officials order Colonel Peckem to go to Evans City to help contain the virus, while scientist Dr. Watts arrives to develop a cure before the virus spreads beyond the small town. Officials also deploy bombers armed with nuclear weapons, to destroy the town if necessary.  answer the following question:  What are the names of the people who served in Vietnam?
A:
David