On Feb. 3, U.N. Ambassador Jeane J. Kirkpatrick boarded a U.S. Air Force jet to begin a 10-day Central American trip that was to have powerful consequences for the Reagan administration.

Announced by the White House only the day before she left, at a time when Vice President Bush was visiting western Europe and Secretary of State George P. Shultz was touring Asia, Kirkpatrick's journey won little notice at the time.

Last week, however, events triggered by her trip exploded in headlines. El Salvador, the administration's most persistent and politically sensitive foreign policy problem, was suddenly being described at the White House as a crisis requiring more immediate military aid and even a possible increase in the number of U.S. advisers and a change in their role.

Once again, this tiny Central American nation had shot to the top of the presidential agenda, generating public and political concern and threatening a new showdown between the administration and important elements of Congress.

Many Americans found it hard to understand what was going on and why.

They had good reason, because the story includes personal and political conflicts, bloodletting in the field and on the battle lines of Washington. Among its elements are bureaucratic struggles over policy, difficult U.S. budgetary choices, the disappointing performance of the Salvadoran army, waning hopes for diplomatic solutions and reports of a growing uneasiness in Central and South America about U.S. fortitude.

Despite all of this, the fundamental direction of U.S. policy has changed little since the last days of the Carter administration, largely because of a belief that other proposed approaches, military or political, are too risky.

The questions under debate within the administration now, as at times in the past, involve tactics, timing and emphasis.

The immediate problem centers on President Reagan's call for Congress to provide an immediate $60 million in additional military aid to keep the Salvadoran military operating at current levels of supply.

This was the reason for his meeting with congressional leaders last Monday, and a second meeting is tentatively being planned for Tuesday.

The chances appear good that, after heated debate, Congress will reluctantly comply with the main elements of Reagan's request. But its price could well be political strings that would limit the administration's room for maneuver and raise further questions about the future direction of U.S. policy.

Another cause for uncertainty, and an element behind the dissonant official statements of the last week, is a struggle between a hard-line faction led by Kirkpatrick, national security affairs adviser William P. Clark and the Pentagon against the State Department's more pragmatic bureau of inter-American affairs, headed by Assistant Secretary Thomas O. Enders, the dominant figure in U.S. policy toward the region since Shultz became secretary of state.

The leak Feb. 10 of a controversial "two-track" policy paper prepared by Enders brought an explosive reaction from an unaware Reagan, according to official sources, and was used by Enders' opponents to "rein in" the towering, blunt-spoken career diplomat whom some accused of being "a Lone Ranger" in policy-making toward Central America.

Among signs of change are more attention to Kirkpatrick's views, appointment of former Florida Democratic senator Richard Stone as a special trouble-shooter on Central American political matters, what an official called "the reintegration" of Enders' bureau into overall administration policy, and closer supervision of the bureau by Shultz and Undersecretary of State Lawrence S. Eagleburger.

Another official said that for most of the eight months after Shultz took office, Central America was "a sixth-floor problem," referring to the fact that Enders and other chiefs of regional bureaus have offices on that floor of the State Department. Clearly in recent days it has moved up to Shultz' seventh-floor domain.

The stresses involved, and Shultz' relative unfamiliarity with the issues, are said to have contributed to his surprising outbursts about El Salvador in recent congressional hearings.

A secret briefing paper prepared for Shultz by Enders' bureau last June 26, the day after Shultz was named to succeed Alexander M. Haig Jr. as secretary of state, maintained that "the trend of events in Central America is now running in our favor."

The paper said that "the Reagan administration inherited a chaotic situation" in the region but that "in the ensuing 17 months this negative trend has been arrested."

In El Salvador, according to the document, "military capabilities have steadily improved." Cited were the "psychological and military disaster" for the guerrillas of the March 28 elections and the "increasingly successful" efforts at interdicting the guerrillas' arms supply.

On the diplomatic front, Venezuela and Colombia were said to "continue their strong support" for the Salvadoran government and "Mexico's unhelpful role is diminishing as that country focuses more on domestic economic concerns."

The briefing for Shultz laid heavy emphasis throughout on the crucial requirement of maintaining congressional support, described as "our principal difficulty" in view of the need for a steady flow of security and economic aid to Central America "at an annual rate of about $750 million for each of the next three years."

If unsuccessful on Capitol Hill, Shultz was told, "we will face increased chances of a political/military debacle in Central America with grave national security consequences."

The concern about a Central American "debacle" began in the Carter administration as the result of the 1979 revolution in neighboring Nicaragua that ousted Anastasio Somoza, an old-line military dictator and U.S. ally, and brought to power the radical Sandinista guerrilla movement, with Cuban support.

The Carter administration, during its final year in office, seized on a military coup by reform-minded Salvadoran officers as a vehicle for avoiding a repeat there of the Nicaraguan experience. It evolved a policy of assisting Salvadoran armed forces in containing the guerrilla insurgency while simultaneously prodding the government, controlled for generations by the military and ultra-rightist land owners, toward the political and economic reforms necessary to provide an alternative to the extremes of left and right.

That has remained the basis of U.S. policy ever since, despite the tendency of the Reagan administration, especially during Haig's tenure, to describe its actions in tough, anti-communist rhetorical terms.

In the late summer of 1981, Haig evolved a plan for a naval blockade of Cuba to stop that country's support of the guerrillas. According to a participant in the crucial White House meeting, Reagan, influenced by Pentagon objections, vetoed the idea.

Last year, the administration also explored Mexican-sponsored negotiations with Nicaragua and Cuba to end those countries' support of the Salvadoran guerrillas.

However, that approach was dropped because Washington became convinced that Jose Lopez Portillo, then the president of Mexico, was tilting too far toward the Nicaraguans and seeking to use the mediator's role to increase his country's influence in the region and the nonaligned movement.

Each time, the administration has returned to the basic approach of seeking to defeat the guerrillas militarily and moving the Salvadoran power structure toward a democratization process that will win popular support.

Despite constant criticism by congressional liberals, the administraion has stubbornly insisted that its policy offers the only hope of preventing a communist takeover in El Salvador.

In defending the administration's approach, Enders and other officials frequently cited Venezuela as their model. The analogy is based on the fact that Venezuela, after a long period of military dictatorship ending in 1958, was able over the next decade to beat back a major, Cuban-supported insurgency and emerge as one of the most stable democracies of Latin America.

Despite the theory of what should happen and such ebullient assessments as last June's paper for Shultz, the facts of life in El Salvador keep getting in the way.

State Department officials now say that Salvadoran reverses date to last October, when the army failed to respond effectively to a major guerrilla offensive.

The attacks coincided with a high point of internal competition between Defense Minister Jose Guillermo Garcia, strongman of the army and government, and retired major Roberto d'Aubuisson, head of the national assembly and the leading right-wing figure.

"The army was polarized, almost paralyzed. It wasn't until November that they got themselves together," an official said. The army's troubles grew in January when the revolt of a field commander, Lt. Col. Sigifredo Ochoa Perez, exposed deep fissures between combat-oriented officers and a Garcia group, said here to be good at administration and reform but to include many "who couldn't fight their way out of a paper bag."

While this was exposing problems in the war zone, Congress last Dec. 20 passed a continuing resolution that left the administration with $26 million in military aid for El Salvador in fiscal 1983, rather than the $61 million requested. The administration decided almost immediately that it would need more money, but until just about a week ago had not seriously addressed how it would obtain the funds from an increasingly restive Congress.

In late January, the administration submitted a supplemental appropriations request to Congress for many overseas programs but did not include El Salvador because, according to a State Department policy maker, "we knew how controversial the issue is." The fear was that other programs abroad would become hostage to a prolonged debate over El Salvador.

In retrospect, says an advocate of the Kirkpatrick-Clark policy line, "this was a very bad mistake because it made it certain that the debate would be focused on this issue later on." Indeed this very debate, with the money issue at its nub, is what erupted at the White House and on Capitol Hill last week.

An extensive review of the situation, motivated by the money problem and by growing unease about El Salvador, began early in January at the assistant secretary level and spread late in the month to the top National Security Council level and involved the president.

Reflecting his rising concern, the president overruled what one official called "the lack of enthusiasm of the State Department" and invited Stone, an advocate of a tough line toward communist activities in the hemisphere, to join the administration as an adviser and special emissary. He also asked Kirkpatrick, whose opinions about Latin America are given careful attention by Reagan and Clark, to visit the region.

Informed sources said Reagan had become concerned by reports that mounting criticism in Congress had caused jitters among U.S. allies in the region. These sources said Kirkpatrick, whose trip was arranged on short notice, was instructed to reassure regional leaders of U.S. steadfastness and to give the president her personal assessment and recommendations.

Her major conclusion, which she revealed in an abbreviated form to congressional leaders at last week's White House meeting, was that the "domino theory," however discredited it might be in this country as the result of the Vietnam war, is regarded by Central American leaders as very relevant.

According to her report, which struck the lawmakers as extremely gloomy, she was told repeatedly that if the guerrillas win in El Salvador, the entire isthmus will soon fall under the control of Cuban-oriented regimes.

Her recommendations:

The administration should do everything possible to provide immediate additional military assistance to El Salvador. This proposal apparently helped to focus the policy debate on the long-discussed issue of seeking $60 million or so in emergency aid from Congress or an administration fund for contingencies.

Expanded economic aid is also vital in El Salvador and elsewhere in the region to relieve pressing humanitarian problems and, on a more long-range basis, to assist in infrastructure development, creation of "consensus-building democratic institutions" and easing of the severe credit crunch inflicted by the world recession.

The United States should interpose no objection to efforts by area governments to bring about a conference including Nicaragua and El Salvador to facilitate solutions to internal and external conflicts in Central America. While supporting such a regional approach, Kirkpatrick recommended that Washington "not try to negotiate with Cuba or Nicaragua for a settlement of the Salvadoran conflict over the heads of the other Central Americans."

In this, she made a distinction between regional negotiations involving area governments and calls for promoting negotiations between the Salvadoran government and guerrilla forces there.

The latter, according to Kirkpatrick, is uniformly opposed by regional leaders whom she saw on her trip because they fear it would result in a sharing of power that leads eventually to a communist takeover.

The controversy over Enders' "two-track" proposal arose in part because of conflicting views and reports of how this distinction would be applied.

State Department sources insist that Enders had in mind rejuvenating a diplomatic track through international maneuvering involving Central and South American states and perhaps Spain.

But his proposal was seen by some high officials as a first step toward internal Salvadoran negotiations.

What is now emerging, and what both Kirkpatrick and Enders are reported to favor, is a meeting of Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua and five "honorable observers"--Colombia, the Dominican Republic, Mexico, Panama and Venezuela--to restart the stalled diplomacy about the troubled region.