One year after his most humiliating defeat within the Reagan administration, Secretary of State George P. Shultz has become the central figure in U.S. foreign policy.

In undramatic fashion, through gradual accretion of authority and steady elimination of rivals, Shultz has become the senior executor and shaper of President Reagan's global policies. Shultz and Reagan, by all accounts, have developed an increasingly warm rapport.

"He is the tortoise who moves ever so slowly, but he just keeps on coming and finally wins the race against the hares," said a highly placed State Department veteran.

Another experienced observer described Shultz as "an unsophisticated thinker about foreign affairs" who tends to simplify, sometimes oversimplify, important issues. "He has none of Henry A. Kissinger's virtues of brilliance, but fortunately he doesn't have Kissinger's vices either . . . . He's low-key, persistent and unextremist." The longer Shultz remains in the job, the official said, the more he is master of the foreign policy process.

Since his successful arms control talks a month ago in Geneva with Soviet Foreign Minister Andrei A. Gromyko, Shultz has become notably more self-confident in public and in the private councils of the administration. In addition to arms control, which has been an area of intense bureaucratic dispute, Shultz has grasped the previously elusive reins of policy in Central America and dominates U.S. policy toward the Middle East and southern Africa.

Through personal attention, Shultz has made an ally of the presidential national security adviser, Robert C. McFarlane.

A secretary of state-national security adviser alliance has been a rarity in Washington since the days when Kissinger held both jobs for 26 months in 1973-5. Now Shultz and McFarlane agree "about 85 percent of the time," according to an insider, which contributes to Shultz's strength.

The most prominent rival remaining is Secretary of Defense Caspar W. Weinberger, who was a longtime associate -- and immediate subordinate -- of Shultz at the Office of Management and Budget and the Bechtel Corp.

Shultz and Weinberger have clashed repeatedly in public but those who have seen them in their weekly breakfast meetings and on social occasions said they have never observed any personal enmity.

To the dismay of some conservative political figures, those who have lost Shultz's confidence have been removed, whether by design or accident, from the foreign policy process.

Shultz's fingerprints did not show up when William P. Clark suddenly left the post of presidential national security adviser in October 1983. But Shultz had become intensely distrustful of Clark several months before when large-scale military exercises were suddenly ordered in Central America without the knowledge of the secretary of state. Their disclosure through a press leak was disastrous to the administration's efforts to sustain congressional support for the antigovernment rebels in Nicaragua.

Shultz fought quietly to prevent Ambassador to the United Nations Jeane J. Kirkpatrick from replacing Clark in the White House basement, considering her part of the problem rather than the solution in complex situations. Kirkpatrick has said little or nothing in public against Shultz, but she has been bitingly critical in private, and friends expect that she will open up on the secretary of state when she returns soon to private life.

The latest figure to tumble -- to the silent swish of one Shultzian hand clapping -- was retired Lt. Gen. Edward L. Rowny, the chief U.S. strategic arms negotiator. Rowny dramatically quit the Carter administration's arms negotiating team in 1979 and has never been regarded at State as a team player.

It was widely assumed that Rowny would keep his job in the new U.S.-Soviet bargaining soon to begin in Geneva. But Shultz is reported to have been irritated by Rowny's bids for prominence at the Jan. 7-8 Geneva talks.

When personnel decisions were being made 10 days later, Shultz recruited former Republican senator John G. Tower of Texas to negotiate strategic arms and suddenly Rowny was a "special adviser" with an unclear charter.

Shultz was expected to be a "take charge" figure when he came to the administration in June 1982 to replace Alexander M. Haig Jr. But after initial successes Shultz found it hard to establish a solid foothold on the slippery slopes of Reagan policy-making.

His position seemed anything but preeminent a year ago yesterday. He learned then, via secure telephone while on a visit to the Caribbean, that Vice President Bush, Weinberger and White House chief of staff James A. Baker III had teamed up in his absence to arrange the pullout of U.S. Marines from Beirut.

Several weeks earlier Shultz had agreed reluctantly to "an orderly, long-term change" in the mission of the embattled Marines, according to an official who was deeply involved in the process. But as the Lebanese army crumbled, Shultz argued that this was the wrong time for any precipitous move -- and he believed that Reagan agreed with him.

Last Feb. 7 Reagan was in Las Vegas making a political speech and Shultz was on a ceremonial visit to the Caribbean island of Grenada. That day the remaining senior figures in the administration decided to cut the losses in Lebanon by withdrawing the Marines.

Shultz sought vainly to reverse the decision in a lengthy "last-ditch conversation" from Grenada with McFarlane.

"It was a very low point" for Shultz, according to an aide. "He never mentioned resigning, but my guess is he mulled it over that night." Both the substance of the pullout decision and the way it was taken cast a long shadow over his position.

The unsuccessful U.S. effort in Lebanon, widely regarded as the most serious foreign policy setback of Reagan's first term, left its marks on Shultz to this day.

The costly terrorist bombing of the U.S. Marine barracks, which shattered the domestic political consensus, was responsible in large part for the secretary of state's uncharacteristic personal crusade against international terrorism. Shultz still holds almost daily meetings on terrorism at which, an aide said, "he pushes, pushes, pushes" and "just pounds away at the details" of terrorist threats and U.S. counteractions.

Shultz's deep reticence about high-profile reinvolvement in Lebanon and, to a degree, the Middle East in general is believed by some close observers to reflect the bitter experience that culminated for him a year ago.

After appeals from the region for renewed U.S. mediation and leadership, Shultz dispatched Assistant Secretary of State Richard W. Murphy last September on a series of fact-finding missions. The No. 1 directive to Murphy in his seven weeks of shuttling among Middle Eastern capitals was to negotiate quietly and avoid the appearance of a new U.S. commitment.

The Murphy missions and the explorations of Ambassador Harry W. Shlaudeman with Nicaragua and surrounding Latin countries reflect Shultz's preference for ultra-discreet diplomacy, with the reins held firmly by the secretary of state in Washington. So do the southern African soundings directed by Assistant Secretary of State Chester A. Crocker, which began under Haig.

A former labor mediator whose collective bargaining experiences were formative, Shultz believes firmly in communications and negotiations. But his strong conservatism leads him to take tough positions that hamper success in negotiations with adversaries.

Shultz was in favor of the U.S. invasion of Grenada and has backed military and paramilitary pressures against Nicaragua. At the same time, Shultz was the originator of bilateral negotiations with Nicaragua, which he kicked off last June in a surprise trip to Managua.

These negotiations were recently suspended by Washington -- at Shultz's suggestion, according to State Department sources -- because of what he viewed as Nicaraguan intransigence on a regional settlement.

Shultz's penchant for negotiations and affinity for the professional Foreign Service have aroused some distrust, despite his generally conservative views.

Last October, for example, national news organizations were informed by telephone calls from Capitol Hill that Shultz, then on a visit to Panama and Mexico, was on the eve of signing a secret four-part deal with Nicaragua.

It was denied by senior officials and didn't happen. State Department officials said they traced the reports to the office of Sen. Jesse Helms (R-N.C.) and Constantine C. Menges of the National Security Council staff.

In late December, conservatives protested that Shultz was preparing to replace their favorite political ambassadors with career officers. The protest does not seem to have changed Shultz's plans for a personnel shuffle, except to create soft-landing jobs for some of those being displaced.

Shultz's trouble is "he hasn't recognized that one of the major obstacles to carrying out Reagan's policies is the State Department," said Burton Y. Pines, vice president of the Heritage Foundation.

Like the man himself, Shultz's concept of his job is simple, self-effacing and undramatic. If asked for his objectives, Shultz will begin in the fashion of a professor -- which he was at the University of Chicago and Stanford -- to cite "the advancement of United States national interests."

If asked about his methods, Shultz speaks of tending the soil of diplomacy as a gardener would -- digging, planting and nourishing relationships with other nations. There is little here of grand strategy, diplomatic brinkmanship or brilliance, but his concept seems to be working for this administration at this time.