Ron Drach flinched as a raucous volley of shots rumbled across the South Lawn of the White House. It was a Colonial Army Platoon, but Drach responded with a reflexive "Incoming" the battle cry of his own war -- Vietnam.

He stood in a group of veterans, virtually at President Carter's feet, under the Truman balcony where Carter and Defense Secretary Harold Brown were seated. "Brings back fond memories" Drach muttered snidely. "That scared the hell out of me. After 13 years, I'm still gun-shy."

Drach, who served with the Army at Tyninah and Chulai, now does battle in Washington as National Employment Director of the Disabled American Veterans. And along with more than 1,000 other representatives of veterans' groups, he attended a White House reception Saturday night celebrating the 50th anniversary of the Veterans Administraion.

Following the reception, the crowd gathered on the South Lawn for a pagent of fireworks, Tchaikovsky's 1812 Overture and re-creations of military scenes from American history, including the colonial-era display. A fife and drum corps paraded; the United States Army Chours crooned "Inky Dinky Parlez-vous" and "Don't Sit Under The Apple Tree With Anyone Else but Me," and the Air Force Drill Team tossed and twirled their m-14 rifles.

"Someday we hope that there will be no war, that there will be no threat of war. No need to call Americans on duty in a time of peace in order to preserve that peace," Carter told the audience. "That time has not yet come. It is still necessary for our nation to have a strong defense. It is for young men and women to demonstrate patriotism in a time of peace in order to prevent war and to prevent death."

Earlier in the day, Supreme Court Justise William F. Brennan had cleared the way for renewed draft registration, which pleased many of the veterans, including Veterans Administration chief Max Cleland, a triple amputee who was wounded during the Tet Offensive in Vietnam.

"The registration effort is primarily an administrative mechanism," said Cleland. "The President has repeatedly said he does not want a draft and had moved in that direction. The country is more supportive of national service and its problems. The world situation has made us more sober about what it takes to maintain our way of life in this world."

Brown said the veterans he spoke to Saturday night were concerned about "the usual." He said, "They're interested in a strong defense. That's why it's always good to be at this kind of a meeting."

As for Republican presidential candidate Ronald Reagan's hard-line defense position, Brown said, "The question is not what the candidates say about defense but what they do. The country is determined to have a strong defense and the Carter administration recognized this need when it came into office."

In from New Cumberland, Pa., Paul Hastings, National Commandant of the Marine Corps League, concurred with Brown. "Budget constraints have strangled national defense," he said. "The all-volunteer force isn't making it. Registration is the first step. Actually what we need is a draft. Carter's finally seeing the light."

Nearby, in the State Dining Room, where guests cued up for white wine and fresh fruit, actor Kirk Douglas, in town plugging his new film, "Final Countdown," said he felt part of the military crowd. The film is about the modern-day aircraft carrier Nimitz, which becomes entrapped in a time warp and appears at Pearl Harbor just before the Japanese attack. "I play the captain of the Nimitz, of course," he explained, drawing his arms to his sides, striking a rigid pose. Douglas also said he is a friend and supporter of Carter's: "I think in a crazy, mixed up world he has kept his cool. He's done a good job."

The other guests, including military chiefs of staff, represented a range of military eras, from Spanish-American War veterans commander Herman Miller, 101, who was wheeled in wearing his Spanish-American War uniform, to Vietnam-era veterans who noted that there were no inspiring battle songs from their fight. "I guess there was Country Joe and the Fish, 'One, two, three, What are we fighting for?" said one, repeating the lyrics of a song that was popular at Woodstock in 1969.

But despite agreement on a strong defense, representatives of various veterans' caused all managed to lead discussion to their own particular concerns, and each complained that his cause had been shortchanged.

Normand Gonsauls, president of the Noncommissioned Officers Association, who passed out business cards in a plastic case including an American flag pin, said benefits have been eroded. "it's like the stump of a tree, every year they chopping and chopping. Soon there won't be anything left."

Jim Pechin of the Center for Community Economics railed at veterans' employment programs: "The Small Business Administration and the treatment of veterans hasn't been bad. It's been nonexistent."

And VA medical investigator Roslyn Yalow, a 1977 Nobel laureate who joined the VA in 1947, said the service provides excellent medical care. "Well, at least they did when I came," she said after a moment's thought. "I've been complaining to a great extent about the appropriations to VA support of research."

Jan Scruggs, president of the Vietnam Veterans Memorial Fund, who was "shot up and everything over there", voiced the concern of many veterans of that war. "Vietnam is far enough behind us now that everyone can pretty much agree it was the veteran who really paid the price of the war. The economy was booming over here, minorities were gaining new rights, and we were over there."

Carter, who praised the VA's new psychological readjustment program, said Vietnam veterans "had an additional burden to bear because our nation was divided about that war, and the deep love and appreciation, the expressions of care when other veterans returned, quite often were not available to Max Cleland and my oldest son, and to many others."

And while the United States Marine Band finished the program with Sousa's "Stars and Stripes Forever," Drach, limping, slipped out of the White House -- before the smoke and explosions of the fireworks ignited in honor of the veterans.