After a day of speech-making in New Orleans, President Reagan stepped out of his helicopter late Monday afternoon and walked into his residence to make the decision on the largest strategic weapons package since the era of intercontinental missiles began.

He called five senior advisers to the second-floor sitting room in the White House. After one hour of discussion, as his advisers looked on, the president signed the paper that made it official: The controversial MX missile will be deployed as scheduled, but it will not be a mobile missile, after all.

Thus ended months of internal debate that had produced sharp divisions within the Pentagon's military hierarchy and the president's civilian inner circle of advisers over how America should enter the next generation of nuclear weaponry.

The men looking on in the White House sitting room -- Defense Secretary Caspar W. Weinberger, national security adviser Richard V. Allen, presidential counselor Edwin Meese III, presidential chief of staff James A. Baker III and deputy chief of staff Michael K. Deaver -- kept the president's decision as secret as the blueprints for the weapons themselves. Vice President Bush and Secretary of State Alexander M. Haig Jr. were informed, but few other administration officials knew of the decision until it was announced Friday.

The option that the president finally chose had fewer advocates in the inner councils of the administration than any of the other three that had been considered. But in the president's view it had fewer basic flaws. In the end, his decision to base the huge, new, once-mobile MX missile in existing Titan silos was, more than anything else, a decision to buy time while still moving ahead on America's strategic aresenal.

"What the president really did," said one senior administration official, "was postpone a decision on the basing mode."

Reagan's decision was dictated largely by the limitations of today's technology, but influenced as well by political reality. The one long-term option that was rejected was mobile land-basing of the MX -- which would have required digging up much of Nevada, home state of his close friend, Sen. Paul laxalt (R-Nev.).

The other two options -- housing the missiles in big transport planes that can stay aloft for days, or digging deep silos and developing an antiballistic missile system good enough to shoot down all or most incoming missiles -- will be studied until a new decision is made in 1984.

Throughout the months of decision and debate, presidential advisers say, it was Weinberger who played the pivotal role in the decision and who proved to be the most influential of the president's advisers.

But another powerful force working to shape the decision on basing the MX was Reagan's profound dislike for President Carter's shell-game method of shutting missiles. During the campaign, Reagan came out strongly in favor of building the huge MX missiles, each of which can carry 10 nuclear warheads, but be ridiculed the shell-game basing plans.

The Reagan's decision is consistent with his presidential campaign rhetoric on the MX. It is less consistent, however, with his 1980 campaign rhetoric about how America has a "window of vulnerability" that must be closed.

The "window of vulnerability" of Reagan's 1980 campaign seems, in a sense, to be going the way of the "missile gap" of John F. Kennedy's 1960 campaign. Last year, Reagan talked at virtually every campaign stop about how increase accuracy of Soviet misiles had made America's missiles vulnerable in thier silos. As president, Reagan has now proposed "hardening" these silos to make them less vulnerable to Soviet missiles. But the new MX missiles, when placed in these silos, will be no less vulnerable than any of the Titan or Minuteman missiles would be in them.

Weinberger, according to several of the participants in the process, began the series of high-level strategy meetings in June with a briefing for Reagan and top White House aides that Meese described as giving "the basic outlines of the issues that has to be decided."

Weinberger's deputy, Frank Carlucci, took part in the briefing, but from the start the Pentagon's military chiefs were kept at arm's length. Did the Joint Chiefs of Staff approve the decision? Meese was asked. "Gee, I don't know. Cap handled that," Meese said in a telephone interview after the decision had been announced.

One member of the joint chiefs kept asking his deputies as the months went by, "What do you hear on the street?"

Weinberger had named an outside group, called the Townes Committee for its chairman, physicist Charles Townes, to give him an independent analysis of the strategic defense questions facing the administration.

There was a second White House meeting in June, Meese said, but the process didn't pick up speed until after the Townes Committee report was received in early July.

"The Townes report was very important," Meese said. Another senior administration official characterized the still-secret report as vital to the process because it "characterized the ((shell-game shuttle)) as not survivable."

A White House National Security Council meeting followed, the first of two large sessions of the issue.

Shortly after, Weinberger made a strong presentation to the president for a new option: an airborne MX, with the missile being fired from converted C5 transport planes.

This set the stage for a new round of infighting. The Air Force, which argued throughout for a multiple-shelter shell game, went all out to discredit the air-based plan. Armed with arguments that the accuracy of MX missiles fired from planes could not be guranteed, the Air Force shot down the airbased plan -- its lone victory in the long debate.

Air Force officials were certain that Reagan would have to accept multiple shelters, but others were equally convinced of the opposite. National security adviser Allen made $100 bets with each of two Air Force generals in early summer of 1980, before Reagan had even won the election, that multiple shelters would never be built.

The State Department also argued consistently for a multiple-shelter scheme, contending that the United States should base some new missiles on its own soil, as Washington is asking Europeans to accept new intermediate-range missiles.

A major event, Meese said, was the expanded National Security Council meeting held Aug. 17 in the president's top-floor suite of the Century Plaza Hotel in Los Angeles. There Air Force chief Gen. Lew Allen had his only opportunity to make his case directly to Reagan.

He had at least partial success. Ten days later, the airborne basing was grounded and Weinberger flew back to the West Coast, this time to bring new options, either a scaled-down shuttle system employing 100 missiles and 1,000 silos or deferral of the basing decision for morde research.

In September, another possible solution gained support in parts of the Pentagon and elsewhere in the government, according to a White House source. This was to scrap the MX in favor of the so-called "common missile," a smaller weapon that could be fired from Trident submarines as well as land silos. The common missile had been rejected in the Carter administration because, one planner has said, it is far smaller than Soviet missiles.

It lost out again, in part, officials said, because it would represent a postponement of misile production, and because the billions already spent on developing the MX would have been wasted.

The common missile was still alive when Carlucci wrote a Sept. 14 memo to the Air Force, but days later, Meese said, the president narrowed his options.

He discarded all possibilities except his final choice and the 100/1,000 scheme, Meese said, and indicated that he was close to discarding the mobile plan as well.

Reagan blocked out the decision in his own mind over the next two weeks. When he returned from New Orleans he was ready.

Weinberger was there with a pile of briefing books and a few final checks were made. One official said they checked "money numbers" for the biggest as well as numbers of weapons.

Reagan signed the order that a National Security Decision Directive be prepared and the process was over. But on the basing of MX missiles, the long-term solution to Reagan's "window of vulnerability" was left to researchers who will seek to unravel what planners call "the unkunks" -- the unknown unknowns.