Monday's military strike against Libya marked a turning point in administration policy after five years of internal debate about how to respond to international terrorism. Officials said yesterday that the attack is likely to prompt further terrorist attacks against U.S. targets that could require additional military operations.

"For the first time we weren't debating whether to use military force against state-supported terrorism but the best way to use it," said an official who participated in the deliberations preceding President Reagan's decision to launch the attack.

Reagan said yesterday that the strike against Libya was "but a single engagement in a long battle against terrorism" and vowed to keep up the pressure on Libyan leader Muammar Qaddafi to end his support for terrorism.

Other officials emphasized that the military strike could mark the beginning of a long and difficult period of conflict. One official said the administration would seek further economic actions against Libya by European allies and would use other military options if necessary.

"We're not just turning our head and going off to other business," the official said. "We realize this could be a tough problem to deal with for a while and we're determined to use the energy and resources necessary to deal with it."

Officials said yesterday that the Reagan turn to a military option had resolved the long conflict between Secretary of State George P. Shultz, who for nearly two years has publicly advocated a military response to terrorism, and Defense Secretary Caspar W. Weinberger who, with the support of the Joint Chiefs of Staff, often had objected to using U.S. military forces in antiterrorist operations.

Yesterday, Pentagon spokesman Robert B. Sims took pains to point out Weinberger's support of the operation and said the secretary was involved in target and route selections even though he was out of the country in the week before the attack. There was "uniform consensus and support" for the operations from Weinberger and the joint chiefs, Sims said.

A longtime friend of Reagan said yesterday that "Cap always knows where Reagan's head is and understood that the president had finally determined to strike back."

Reagan, in a speech to the American Business Conference, said U.S. forces "spoke to the outlaw Libyan regime in the only language that Colonel Qaddafi seems to understand."

Reagan arrived at this position as part of a long process in which he was increasingly frustrated at the failure to influence or deter Qaddafi by diplomatic or economic means, officials said. One official said that as Reagan's frustration grew, he more frequently expressed his "revulsion" at Qaddafi's willingness to kill innocent civilians in his efforts to strike against the United States.

But it was the rare success of a military antiterrorist operation -- not frustration -- that apparently convinced Reagan to follow this course, the official said. He said the president's commitment to a military response against terrorism dated to last October when U.S. planes intercepted the hijackers of the Italian luxury liner Achille Lauro, on which a U.S. citizen was murdered.

Reagan's turn to a military response reflects in part his growing reliance on Shultz, officials said. But they said Shultz's position was enhanced by a growing appetite in the White House national security office for military retaliation.

"When Shultz first made his case, the national security adviser William P. Clark sided with Weinberger," an official said. "When Clark was replaced by Robert C. McFarlane, Shultz gained an important ally, but one who believed in a step-by-step approach with military action as the last resort. Now, there is a national security adviser John M. Poindexter who turns readily to military force."

Libya has been a thorn in the administration's side since the early days of the Reagan presidency. On Jan. 30, 1981, days after Reagan took office, the Central Intelligence Agency circulated what it considered to be an important intelligence paper titled "Libya, Aims and Vulnerabilities," which forecast that Qaddafi's "aggressive policies will pose a growing challenge to U.S. interests in the Middle East and Africa." It concluded that Qaddafi would engage in "more adventurism."

On May 6, 1981, in the administration's first confrontation with Qaddafi, the United States closed the Libyan diplomatic mission in Washington and alleged "Libyan provocations and misconduct, including support for international terrorism."

This was followed by the Aug. 19, 1981, exercise in the Gulf of Sidra that led to the downing of two Libyan jets after a brief dogfight with U.S. planes. Soon afterward, the White House received intelligence reports that Qaddafi was sending out "hit squads" to assassinate Reagan and other top administration officials.

Late in 1981 a memo was sent to the president -- signed by then-secretary of state Alexander M. Haig Jr., director of central intelligence William J. Casey and Weinberger -- saying that, at a National Security Planning Group meeting on Nov. 30, "you directed that plans be developed for a military response against Libya in the event of a further Libyan attempt to assassinate American officials or attack U.S. facilities."

These three advisers -- in language remarkably similar to the rationale the administration is now using for its Libyan operation -- recommended that Reagan order contingency military planning "to carry out military action against Libya in self-defense following a further Libyan provocation."

The Pentagon did top-secret planning and proposed a series of graduated responses as contingencies. But they were never carried out, and administration officials subsequently acknowledged that concern about "hit squads" on American soil may have been overblown.

Libya was a back-burner concern during 1982 and much of the following year. But terrorism abruptly became a major administration concern after a suicide truck bomber hurtled into a Marine headquarters barracks in Beirut on Oct. 23, 1983, and killed 241 U.S. servicemen.

"Reagan took that catastrophe very hard," said a longtime associate of the president, "even though the Grenada invasion the following week softened some of the impact. It began to change his thinking."

The Beirut incident also had an impact on two former Marines, Shultz and McFarlane. A week before the attack, McFarlane had replaced Clark as national security affairs adviser. The Marines were withdrawn from Lebanon early in 1984, but terrorism remained of major concern to Reagan, Shultz and McFarlane and was heightened by a Sept. 20, 1984, terrorist attack on the U.S. Embassy annex in Beirut.

On Oct. 25, 1984, in a speech titled "Terrorism and the Modern World," Shultz called for a new and violent response to terrorist activity.

"We must reach a consensus in this country that our responses should go beyond passive defense to consider means of active prevention, preemption and retaliation," Shultz said. "Our goal must be to prevent and deter future terrorist acts, and experience has taught us over the years that one of the best deterrents to terrorism is the certainty that swift and sure measures will be taken against those who engage in it."

But Shultz's counsel, resisted in specific instances by Weinberger and the joint chiefs, did not immediately prevail. Instead, the president agreed to a covert operation in which the CIA would train and support counterterrorist units in the Middle East.

This ended disastrously on March 8, 1985, when a unit of the Lebanese intelligence service, acting without CIA authorization, undertook the car-bombing of a suspected terrorist that failed to kill him but left 80 civilians dead and another 200 injured. As a result, Reagan and the CIA quickly canceled the covert operation.

Three months later, when TWA Flight 847 was hijacked, administration conflicts about how to deal with terrorism remained unresolved. But terrorism quickly became a highly visible issue for Americans during the 17 days of this crisis.

Although a Navy diver aboard Flight 847 was killed by the hijackers, the rest of the passengers were released unharmed. But McFarlane and other high officials concluded in the aftermath that the administration had been fortunate to have the incident resolved peacefully. They privately launched nine months of top-secret planning that culminated Monday in the military operation against Libya known as "El Dorado Canyon."

Sources said it was the TWA 847 hijacking that brought anti-Libyan and antiterrorist policies together. The top-secret planning led to a number of military options in which Libya increasingly was singled out as the main target, even though several other nations, notably Syria and Iran, were also considered to be centers of state-supported terrorism. The plans included a CIA covert operation to undermine Qaddafi and secret military contingency planning with Egypt for a possible joint U.S-Egyptian attack on Libya.

A senior official said yesterday that Libya has been a significant concern for the United States ever since the antiterrorist contingency planning began last summer. But this planning was sharply accelerated by the Achille Lauro hijacking and espcially by the Dec. 27 terrorist attacks at the Rome and Vienna airports.

The airport attacks provoked a furious personal denunciation of Qaddafi by Reagan, who at a January news conference called the Libyan leader "a barbarian." Reagan's concern and Shultz's conviction that force must be used against terrorism finally prevailed in the bombing operation carried out Monday -- an exercise that administration officials see not as the end of an effort to appease terrorism but as the beginning of a long and probably violent conflict with Libya.