IN MARCH 1981, just two months after President Reagan took office, Under Secretary of State Walter Stoessel Jr. went before a Senate committee with a request for $25 million in military aid to El Salvador. Stoessel assured the senators that the request did not foreshadow any big escalation of U.S. commitments in Central America.

"We are doing our best to insure that a similar situation does not develop in El Salvador with what happened in Vietnam," Stoessel said. "We do not foresee the necessity for increases."

Since then, however, American military commitments in Central America have increased, in an exorable process of small and often quiet steps which, when taken together, add up to a dramatic escalation of U.S. military involvement in the region. U.S. military aid to El Salvador, for example, reached $196 million in fiscal 1984 alone. And this has been accompanied by a steady expansion of direct U.S. support and of the U.S. military presence throughout the area.

Neither Democrats nor Republicans are saying much about the Central American issue in their campaigns so far. But the looming question remains: What will happen after November's presidential election? While no one can answer that definitively, it is worth examining the history of the last four years now, in order to understand the direction in which the nation has been moving, and to grasp how much has actually changed since the Reagan administration took office.

Since January 1981, the amount and sophistication of U.S. equipment being supplied to both El Salvador and Honduras has risen; military bases have been built and expanded throughout Honduras, and U.S. naval flotillas patrol the waters off the Central America coast. In El Salvador, American adviser currently operate in combat zones rather than in rear echelon areas, to which they were once restricted.

At the same time, what began as a small- scale effort to interdict arms shipments from Nicaragua to the Salvadoran guerrillas has escalated into an undeclared war against Nicaragua, fought by a "contra" army trained, equipped and financed by the United States. The Central Intelligence Agency has acknowledged mining Nicaraguan ports and contra forces have attacked fuel installations inside the country.

Along with the increasing U.S. involvement have come glowing assessments of progress toward the administration's goals of defeating the Salvadoran guerrillas and toppling the Sandinista government in Nicaragua.

In May 1983, for example, CIA Director William Casey told members of Congress that the contra forces fighting out of bases in Honduras had a good chance of toppling the Sandinista government in Nicaragua by the end of the year. As for El Salvador, the senior U.S. military adviser in El Salvador said in June 1982 that the guerrillas would be "reduced to banditry" within two years.

At the same time, however, U.S. estimates of what it would take to do the job have been steadily revised upward. U.S. Embassy officials in El Salvador advised Washington in June 1981 that the guerrillas could be defeated if the Salvadoran army, including the security forces, were roughly doubled, to 23,000 troops. Five months later the assessment of what was needed for a military victory was upped to 41,000 troops. Today the Salvadoran armed forces number 45,000. But victory still remains elusive, just as the toppling of the Sandinista regime by the contras also seems remote.

Prior to 1979, U.S. military involvement in El Salvador was negligible. It was not until after the Sandinista victory in Nicaragua in July 1979 that the U.S. begin to look more closely at events in El Salvador. The first steps of the escalation began in earnest during the final days of the Carter administration, when it approved $5 million in emergency military aid, supplied the Salvadorans with six helicopters and dispatched about 20 advisers in response to a major guerrilla offensive. Within weeks after arriving in Washington, the Reagan administration asked for an additional $25 million in military aid and a doubling of the number of advisers. In the next three years, despite Stoessel's assurances to the Senate Foreign Relations in March 1981, U.S. military aid rose sharply: to $82 million in 1982, another $81.3 in 1983 and then to $196 million in fiscal 1984.

Similarly, the administration revised its stand on the advisers. When Sen. Joseph R. Biden (D-Del.) of the Senate Foreign Relations Committee asked in March 1981 how long they would be there, he was told by Gen. Ernest Graves, director of the Defense Security Assistance Agency: "Months or at most one year" after training one battalion.

"What specific steps are being taken in El Salvador that are different from those taken in the early days of Vietnam?" Sen. Larry Pressler (R-S.D) asked during the same hearings. Gen. Graves answered that the "most important step" was that the U.S. military personnel in El Salvador would be confined "strictly to training activities" in "the most secure areas." He stressed that the advisers would not "accompany combat operations."

These assurances were repeated a few months later when Assistant Secretary of State for Inter-American Affairs Thomas O. Enders appeared before the House Inter- American Affairs Subcommittee.

Under the War Powers Act, enacted over President Nixon's veto in 1973, the president is required to notify Congress if he sends combat-equipped troops into places where there are, or might soon be, hostilities. The Reagan administration did not file a War Powers report when it dispatched the advisers, and the Defense Department realized this presented problems.

"It is contradictory to be stating that a War Powers report is not required when military assistance is being justified publicly as a result of a major offensive marked by intense, widespread, daily attacks draining the military resources" of El Salvador, the Defense Department's general counsel's office wrote in an internal memorandum dated Feb. 12, 1981.

Nevertheless, the Defense Department counsel argued, it might still be legal not to file a War Powers report "based on the representation that" U.S. military personnel "were not expected to be the subject of a military operation," "would not deploy to hostile areas to advise" and "would not receive hostile fire pay." The lawyers also noted that the advisers would carry only personal side arms, "but would not carry or operate any other weapons."

These "representations" and assurances were, however, subsequently breached.

The U.S. advisers did not come home after one year, as the administration said they would. They are still there. And there have been up to as many as 80 in the country at one time, exceeding the 55 that the Reagan administration had informally agreed with Congress as the maximum. Nor did the advisers stop after training one battalion in El Salvador, the Atlacatl. They trained a second one in El Salvador, the Atonal. Then, 900 young Salvadorans were brought to Fort Bragg, where the Ramon Belloso battalion was formed and trained.

In addition, some 500 Salvadoran cadets came up to Fort Benning for officer training. And more Salvadoran battalions have been trained at a military base established in Honduras.

Nor have the advisers stayed out of military operations, as the administration said they would. A number of U.S. news organizations reported on and photographed the presence of U.S. advisers in June 1983 when the Salvadoran army launched the largest military operation of the war -- a sweep through San Vicente and Usulutan provinces using reinforced units.

American pilots were also involved in combat situations as often as twice a month in 1983, according to a U.S. military official in El Salvador. Moreover, American advisers currently operate with artillery units and several combat units, including one at San Francisco Gotera, a major government outpost surrounded by guerrilla forces.

The advisers have received extra pay for being under hostile fire, according to a report by the General Accounting Office. And they have frequently been in "hostile areas." They were present, for example, at Ilopango Air base when it was sabotaged by guerrillas in January 1982; 17 Army and five Navy advisers personnel were at the garrison in San Miguel when it was attacked in November 1983 and again in March of this year, U.S. government officials have acknowledged.

In 1982, when a Cable News Network television crew filmed five advisers carrying automatic rifles, the senior officer was ordered out of the country, and the others received verbal reprimands for violating rules against carrying weapons other than sidearms. But earlier this year, when the CBS Evening News showed films of advisers carrying MP-5 submachine guns, no disciplinary action was taken. The rules had been quietly changed.

Just as the role of the U.S. advisers has steadily expanded, so has the amount and sophistication of equipment that the United States has supplied. During 1981, the Salvadoran army received approximately half a dozen Huey helicopters. Then, in early 1982, the administration began shipping 12 more Hueys, along with four C-123K troop transports, four O-2A forward air control spotters and eight A-37B Dragonfly jets, from which the Salvadoran Air Force drops 500-or 750-pound bombs. From the $70 million supplemental military aid approved by Congress for El Salvador last month will come a dozen more helicopters.

In addition to increasing shipments of equipment and more money, there has been an escalation of direct U.S. military support. In 1983, U.S. pilots flew reconnaissance missions from their bases in Panama. This past February, members of the U.S. Army's 224th Military Intelligence Battalion began flying OV-1 Mohawk surveillance planes out of Palmerola Air Base in Honduras. The flights were in direct support of Salvadoran army combat operations, marking "the most direct involvement in El Salvador's civil war so far by a U.S. military mission whose scope and size have steadily expanded in small steps over the past year," The Washington Post's Edward Cody reported in March. The 224th Battalion went home to Hunter Army Airfield in Savannah, Ga., in May, but returned to Honduras last month.

Paralleling the escalation in El Salvador has been the military build-up in Honduras. Military aid to that country has increased steadily: $4 million in fiscal 1980; $8.9 million in 1981; $31.2 million in 1982; $37.3 million in 1983; $77.5 million in 1984.

Much of the build-up was disguised behind large-scale maneuvers involving U.S. Marines, Navy, Air Force and Army, which began with Big Pine I in January 1983. That was followed by Big Pine II, involving 12,000 troops; then Grenadero I, and currently, bigger Focus '84. The administration calls the maneuvers "exercises," designed for training purposes, and has said that the bases being utilized were temporary. "Nothing will be left behind," the former head of the U.S. Southern Command in Panama, Lt. Gen. Wallace Nutting, assured Rep. Michael D. Barnes (D-Md.), chairman of the House subcommittee on Western Hemisphere Affairs, prior to the first exercise in 1983.

Yet at Palmerola Air Base near the central Honduran town of Comayagua, U.S. Army engineers put up 132 structures to serve as barracks, offices, a PX and mess halls, and also constructed a road network and fuel storage facilities, according to a recent report by the General Accounting Office which was prepared in response to a request by Rep. Bill Alexander (D-Ark.) for an examination of Defense Department activities in Honduras.In addition, the runway has been improved and lengthened to 8,000 feet, making it suitable for U.S. attack and fighter aircraft, such as F-16s and A-7s.

A second base camp has been constructed near Trujillo, several miles south of the Honduran port of Puerto Castillo and 10 miles west of the base where Salvadoran soldiers are being trained. At Trujillo, Navy Seabees built "Camp Sea Eagle," a complex of 40 barracks, offices and mess halls. In addition, the Seabees extended the length of the airstrip, making it capable of handling U.S. C-130s, constructed a helicopter pad and a ramp for offloading ships at Puerto Castillo, and more than five miles of roads in the vicinity.

Although all this construction was part of Big Pine II, the GAO reported that "it is clear that a more extended use was also contemplated by DOD." (Deleted from the GAO report made public were a couple of lines of "classified material," which presumably explain what the "extended use" would be.)

In the southern part of Honduras, at the port town of San Lorenzo, a U.S. Army engineer battalion again expanded an airstrip to make it able to handle C-130s, and put up 94 buildings. "Fulfillment of (Big Pine II) exercise requirements was not the only purpose for such construction," the GAO notes. (This statement is again followed by deletions, of approximately 20 lines.)

Finally, in eastern Honduras, at Aquacate, another base camp was constructed. U.S. Army engineers lengthened the airstrip, from 4,300 feet (longer than the 3,500 feet needed for C-130s) to 8,000 feet, long enough to accommodate gigantic C-5As.

The United States has also built airstrips at Cucuyagua, near El Salvador, and Jamastran, on the Nicaraguan border, as well as a radar station at Cerro la Mole in south central Honduras that is permanently manned by U.S. Air Force personnel, according to the GAO.

The Reagan administration is now seeking $4.3 million for additional military construction in Honduras during fiscal 1985. Any doubts about the intended permanence of the U.S. presence have been dispelled by the Defense Department. The facilities at Comayagua, which are to be expanded with the additional funds, are currently "acceptable for short stays," but "are unacceptable for indefinite deployments," according to a Defense Department document.

Honduras itself plainly does not need all this military aid. It is one of the most peaceful countries in the region. No significant revolutionary or guerrilla groups operate there. So it is difficult to draw any conclusion other than build-up in that country is not only part of the overall effort against El Salvador's guerrillas, but is also part of the escalating war against Nicaragua.

In December 1981, the Reagan administration authorized spending $19 million to create a 500-man paramilitary force whose function, administration officials said, was to stop the flow of arms from Nicaragua to the Salvadoran insurgents. Since then, according to intelligence officials, the amount spent has ballooned to $150 million and the contra army has grown to between 12,000 and 15,000. It is well-supplied, well-trained and well-paid, with uniforms, sturdy combat boots, canteens, flashlights, compasses and automatic rifles, with U.S.-supplied helicopters to back up ground forces.

Congress has attempted to check the escalating war against Nicaragua, by refusing to approve recent administration requests for funds for the contras. But no one expects the contras to be disbanded or for the war to end soon.

Contra leaders have said they have enough supplies and funds to continue fighting for several months. Moreover, the Reagan hopes to get another $28 million in funds for the contras as part of the continuing resolution to be voted on before Congress recesses early next month.

Even without CIA aid for the contras, the United States has quietly constructed an impressive array of bases and airfields from which U.S. forces could move against Nicaragua if they so desired. The Honduran bases at San Lorenzo and Aguacate are, respectively, 40 and 50 miles from the Nicaraguan border. The airstrip at Jamastran is just a few miles from the border. Moreover, a naval flotilla, including the aircraft carrier John F. Kennedy, an 80,000-ton warship carrying 85 combat aircraft, which took part in the operations off Lebanon, was recently dispatched to the Caribbean waters off Nicaragua, the latest of a series of naval maneuvers in the area.

Though the Reagan administration is seeking to keep things quiet in Central America during the election campaign, it has indicated that it will escalate efforts there after November. U.N. Ambassador Jeane Kirkpatrick, a principal architect of the administration's policy in Central America, told the Republican National Hispanic Assembly last week that the Reagan administration, if re-elected, would step up aid to the contras.

As for the Democrats, Walter Mondale has said that he would end the war against Nicaragua within 100 days after taking office. He has not, however, been specific about what he would do in El Salvador and Honduras.

Up to now, Congress, including the Democratic-controlled House of Representatives, has generally gone alone with the widening of U.S. military commitments.

In looking to the future, it is well to remember that "the past is prologue." Warnings about the nation's direction in Central America are not just from the relatively recent history in Vietnam, when generals and presidents, Democrat and Republican, repeatedly insisted there would be no wider involvement and that the war could be won if the South Vietnamese army only was given enough time, training and equipment. It is also the history of the past four years in Central America.

In spite of all the money, materiel and training -- overt and covert -- that the United States has provided its surrogate armies during the last four years, no one is talking (at least not very loudly) about the "light at the end of the tunnel."