The Defense Department paid outside contractors, including a group of retired generals and admirals, $890,000 over the last three years for studies to help overcome congressional opposition to new chemical weapons, according to Pentagon officials.

"We felt compelled to go for a stronger case" in what eventually became a successful campaign to persuade Congress to reverse a 16-year prohibition on production of nerve gas, said Thomas J. Welch, deputy assistant to the secretary of defense for chemical weapons. Welch said his office initiated the overall contract in 1983 to answer questions raised by legislators and a General Accounting Office report critical of the Pentagon's chemical program.

By law, none of the Pentagon's funds can be used "in any way directly, or indirectly, to influence congressional action on any legislation or appropriation matters pending before the Congress," according to the Defense Department appropriation act. In this case, a Pentagon spokesman said, the effort made through the $890,000 contracts to persuade Congress to build chemical weapons was not legally "lobbying," since the information the contract produced had been requested by Congress.

The contract was given to the Institute for Defense Analysis, a Pentagon think tank in Virginia.

The institute, in turn, hired three subcontractors including Burdeshaw Associates, Ltd., a Bethesda consulting firm with a roster of more than 170 retired senior military officers.

Burdeshaw, with a $240,000 subcontract from the institute, ran a nine-month study using 21 retired U.S. generals and admirals led by Gen. Frederick J. Kroesen, who had just retired as commander-in-chief of U.S. Army forces in Europe. They drafted operational plans for a Warsaw Pact attack on North Atlantic Treaty Organization forces in 1990, based on current Pentagon planning and estimates of Soviet weapons programs. They also suggested a NATO defense plan.

The Kroesen study concluded that if the United States failed to build new offensive chemical weapons, the Soviet Union would be "militarily foolish" not to initiate use of nerve gas against NATO forces in some future European war.

The Kroesen study included several fictional vignettes "which might occur in a war," according to an official summary. One "describes the situation at a naval base in southern Europe at the time the base is struck by enemy air and missiles launched from Libyan bases."

Petty officer Sam Barnes, an air controller at the NATO installation, sees explosions. "A flashing light outlined the small group near the tower. Now they were writhing on the ground and each man seemed to be gasping for air. 'My God, it must be gas! A chemical attack!' . . . . "

Barnes is overcome but "in his final moments of consciousness," according to the study summary, he saw "fire and smoke were everywhere, and the gas emanating from the chemical warheads had begun to take a terrible toll. Air operations ceased."

A $70,000 second phase of the Kroesen study included preparation and presentation early last year of a declassified version of the report to Congress, according to retired Col. Everett Dees, business manager of Burdeshaw. However, an aide to the Senate Armed Services Committee said the committee members wanted a declassified version of the original Kroesen study, something the Pentagon -- at least theoretically -- could have provided without authorizing the $70,000 second phase of the contract with Kroesen and his colleagues.

The second, declassified Kroesen report was circulated among members of Congress last year as part of the major administration lobbying effort to resume production of chemical weapons. Welch said the study "was one of the many pieces that convinced the House" to change its view last year and vote some production of chemical weapons beginning in 1987.

In 1981, 1982, and 1983, Congress refused to end a U.S. moratorium on producing new chemical weapons, begun in 1969, despite Pentagon testimony that the Soviets were building up their chemical weapons capability. Welch said that Congress, in effect, had said, "You haven't convinced us; we are not giving you any money."

A Pentagon spokesman said the former generals and admirals, most of whom had dealt with chemical weapons issues while in service, were chosen to do the study because "they had experience." However, he added, while on active duty "none of the gentlemen independently looked at the interoperability problems" caused by their services being confronted simultaneously with a Soviet chemical attack.

He said the study was an "eye-opener" for the retired officers and has been "accepted as the baseline for discussing the chemical weapons problem."

Asked why such a study had not been performed by active-duty officers, the spokesman replied, "The demands on active-duty senior officers do not permit their concentrated study of this single issue in-house."

In the report, the retired officers brushed aside what they listed as the disadvantages for the Soviets in employing chemical weapons.

They said the most serious threat, a NATO response with nuclear weapons, could be overcome with a "well-planned, well-executed offensive." As for worrying about any NATO retaliation with chemical weapons, Kroesen said, "If we were military commanders of the Red forces, we would accept the casualties . . . as just part of the battle."

Concern for "the world's repugnance to employment of weapons long touted to be immoral and disgusting," the report said, would be of "little consequence" because of the "history of Soviet lack of concern for world opinion."

In echoing Pentagon policy, the study said U.S. resumption of chemical weapons production was "essential to assure the absence of chemicals from the battlefields on which our forces may be engaged in the future."

The "final finding" of the study was that the billions of dollars being spent for modernization of NATO conventional and nuclear forces are "hostage to the absence of a companion program modernizing our ability to survive and fight in a chemical environment."