Career soldier Donald Paquin survived two combat tours in Vietnam and 21 years of tough duty for the U.S. Army. But when he became a private businessman it was that same well-served army, he claims, that intentionally did him in.

After retiring from the service as a major, Paquin turned to the quiet life of a restaurateur. Settling in Lawton, Okla., he opened a Taco John's franchise just a mile or so outside the front gate of the enormous Fort Sill army base. If there was one thing GIs loved, Paquin knew, it was fast food.

Right from his grand opening in April 1984, his enterprise looked like a winner. The troops poured in for so many tacos that he and his family kept the place open for long hours, seven days a week. By October 1985, business was booming so strongly that he borrowed $67,000 to add another dining room. His future as a small-business success looked secure.

But in the spring of last year, just a few months after the $67,000 had been spent, events took a nasty turn for the highly decorated former Green Beret. First, the Fort Sill commander, General Eugene S. Korpal, declared that soldiers would no longer be permitted to dine in Lawton restaurants while wearing fatigues -- the work uniform worn by the vast majority of the installation's more than 20,000 troops. Then, three months later, the fort opened its very own Burger King.

The presence of the Burger King was not in itself unusual. The Army and Air Force Exchange Service (AAFES), a military retailing operation, already runs some 80 Burger Kings on bases in the United States, Europe, and the Far East, with plans to build about 100 more. No, what really galled Paquin was Korpal's dictum on uniforms. It effectively cut the troops off from their supply of off-post tacos, and as the GIs disappeared from Taco John's, so did Paquin's business base. He shut the place down last March.

"General Korpal and the AAFES destroyed over three years of my work," he says bitterly. "They stole over $100,000 from me -- everything I went into business with is gone."

His theory -- which the army and the AAFES deny -- is that Korpal's action was aimed at forcing For Sill soldiers to spend their fast-food bucks on base. Why? The AAFES-run Burger Kings give 50% of their earnings to the military to help build leisure and recreational facilities, such as tennis courts and photo labs. And the contribution is substantial. AAFES spokesman Gary Haynie says a military Burger King can bring in $225,000 a month -- $2.7 million per year.

But if Paquin learned anything in the army, it was how to fight back. Enraged by Korpal's move, he has taken his case by mail to his senators and congressman, to Attorney General Edwin Meese III, and finally to President Reagan. In his letters, he alleges that Korpal conspired with AAFES to restrain free trade artificially. He suggests that the general be court-martialed for violating the Sherman Antitrust Act.

Although he's probably fighting in vain, he has at least raised the question of why the U.S. military, through AAFES, should be running retail operations in direct competition with the private sector.

AAFES is a big-league organization, with 72,000 employees and nearly 16,000 facilities in 23 countries; last year it brought in $5.2 billion in revenues. Its empire embraces everything from mammoth post exchanges to barbershops and snack bars. And it touches businesses located anywhere near military bases. When the AAFES began selling personal computers at Fort Sill, Shawn Swanson, manager of ComputerLand of Lawton, saw sales of the Apple IIc, his most popular model, decline by 50%. Its Fort Sill Burger King has already sapped $400,000 in sales -- and $150,000 in profits -- from two off-post Burger Kings, says off-post proprietor Al Trueblood.

Of course, given that AAFES burger stands, operating on federal land, pay no rent, no land costs, no property taxes, no sales taxes, and no income taxes -- well, their prices are tough to match.

As for Paquin, he calls AAFES "a tax-free monopoly that's mounting a growing attack on the free enterprise system," the vehicle by which the army sabotages the very economy we pay it to protect. But he sees no end to it. "They keep talking about the welfare of the troops, and they've got the American flag out in front of them, so it's a tough nut to crack."

With his Taco John's shuttered, he and his family are pulling out. And wherever he decides to set up shop again, he says, one thing is clear: it will be as far from a military base as he can get.