In THE GROWING debate over relations with Moscow, the Communist coup in Afghanistan is widely cited as one of the cardinal examples of a new Soviet aggressiveness in the Third World. Yet closer examination shows that it was the shah of Lan, not Leonid Brezhnev, who triggered the chain of events culminating in the overthrow of the Mohammed Daud regime.

This is important to recognize, because a simplistic interpretation of Soviet strategy in South Asia could lead to unrealistic - and self-defeating - American responses to the problems now unfolding in the region.

Put in perspective, the 1978 Afghan coup emerges as one of the more disastrous legacies of the shah's ambitious effort to roll back Soviet influence in surrounding countries and create a modern version of the ancient Persian Empire.

Throughout the Cold War, Afghanistan, which shares a 1,050-mile border with the U.S.S.R., had pursued a Soviet-tilted brand of neutralism reflecting its vulnerable, land-locked position. The Russians, in turn, had kept up friendly relations with successive Afghan rulers, and they were particularly pleased with Daud's cooperative attitude following his overthrow of the monarchy in 1973. Beginning in 1974, however, Iran, encouraged by the United States, made a determined effort to draw Kabul into a western-tilted, Tehran-centered regional economic and se curity sphere embracing Pakistan, India and the Persian Gulf states.

The most visible results of this Iranian campaign were in the economic and cultural arenas. Under a $2 billion, 10-year economic agreement with Afghanistan in 1974, Iran was scheduled to replace the Soviet Union as Kabul's biggest aid donor.

A projected rail and highway network, linking Afghanistan to Persian Gulf ports, would have ended Afghan dependence on Soviet trade and transport outlets. Tehran Radio stepped up its broadcasts in Dari, the Afghan variant of Persian, and Iranian publications flooded the Afghan market.

Among the less visible aspects of the shah's offensive was a busy Savak station, competing with the well-established KGB, which fingered suspected Communist sympathizers throughout the Afghan government and the military. In particular, the shah's secret police wanted the removal of members of the parcham (Red Banner) party, one of the two pro-Soviet Communist factions in Afghanistan. The Parchamites had helped Daud win power despite his ultra-conservatism on domestic issues.

In September 1975, prodded by Tehran, Daud dismissed 40 Soviet-trained officers from the armed services. He then moved to reduce future Afghan dependence on officer training in the U.S.S.R. by initiating training arrangements with India and Egypt. Most important, in Soviet eyes, Daud gradually broke off his alliance with the parcham Communists, announcing that he would start his own National Revolutionary Party and would ban all other political activity under a projected new constitution.

A collision course.

Recently uncovered evidence makes clear that Daud's decision to break with the parchamites, coupled with his pro-Tehran drift, provoked a significant change in Soviet Policy toward the Afghan Communist movement during the course of 1976. Until then, Moscow had shown little concern over the debilitating split between the parcham faction and its rival, the Khalq (Masses) party, which opposed the alliance with Daud.

But in March 1976, the Iraqi Communist daily Tariq Al-Shaab, then the principal mouthpiece for Moscow on Afghan matters, published an unprecedented appeal for Communist unity in Afghanistan. It offered, in effect, to recognize Khalq leader Nur Mohammed Tazaki as the leader of a unified party if he would come to terms with parcham leaders. This was the opening gun of a Soviet effort to orchestrate a merger of the two parties that finally proved successful in May 1977.

On visits to Tehran and Kabul in early, I found numerous indications of the confrontation then shaping up between Iran and the U.S.S.R. In Tehran, high Foreign Ministrky officials spoke confidently of the leverage that Savak was exercising on the Daud regime. Iranian aid, these officials said, had been conditioned on a continuing crack-down against both parcham and Khalq, accompanied by an Afghan peace agreement with pakistan ending Afghan support for Communist and other insurgent groups in pakistan's Baluch and pathan tribal areas.

In Kabul, president Daud explained to me that Iran's new economic potential had altered the geopolitical equation in the region, offering an alternative to excessive dependence on Moscow. "Our historical relations with Iran were unpleasant," he said, "but we must adapt to the new realities."

Told of this conversation, British Ambassador to Afghanistan Roy Crook, a veteran Afghan specialist, predicted that "if it goes too far and too fast," Tehran's diplomacy "will surely upset the Russians and produce a reaction." The Soviets were beginning to give some help to the Afghan Communists, he said, in order to keep Daud in line and prepare for an increasingly uncertain future. But "they are still generally satisfied with the degree of influence they have, provided that the drift to the right does not go too much further."

In the year that followed this prescient warning, Daud rapidly accelerated his shift to the right in both domestic and foreign affairs. Armed with his new one-party constitution, adopted in February 1977, he reahuffled his cabinet and named a hard-line interior minister, Abdul Qadir Nuristani who intensified repression of the Communists and other opposition elements. He intensified his efforts to expand military training arrangements with India and Egypt, dispatching a former army chief of staff on visits to Delhi and Cairo. Despite nationalist charges of a sell-out, he announced his intention to ratify a bitterly controversial, long-stalled treaty with Iran dividing the waters of the Halmand River, a key link in the shah's dream of a Tehran-centered regional economic union. In March 1978, he concluded the peace agreement with Pakistan sought by the shah, pledging to oust Baluch and Pathan guerillas who were using Afghanistan as a sanctuary. Pakistan agreed, in turn, to train Afghan officers. In early April, much to the distress of the Russians, Daud went to Egypt and Saudi Arabia. The shah was scheduled to visit Kabul in June, and Daud was preparing for a White House meeting with President Carter in September in which he was expected to seek greatly upgraded U.S. economic aid.

It was at this juncture that Afghan Communist leader Mir Akbar Khaiber was murdered on April 17, 1978, touching off the final showdown between the Communists and the Daud regime. By all indications, the murder was directly or indirectly arranged by Interior Minister Nuristani, who had told a number of friends that it was time to "finish off" the Communists before they got too strong. On April 24, the seven top Communist leaders in the country were arrested; on April 26, hundreds of suspected Communist sympathizers were purged from government posts; and on April 28, Daud was assainated.

Did the Soviets assist or encourage the coup? While nothing but circumstantial evidence of a Soviet role has so far surfaced, it is difficult to believe that there was no Russian involvement, given the extent of the Soviet military and intelligence presence in Kabul. The unusually effective precision bombing of key targets during the critical moments of the fighting strongly suggested the use of Soviet pilots. Still, assuming Soviet involvement, it was Daud who forced the issue with the Khaiber murder and the subsequent roundup of the party leadership.

Significantly, there is widespread agreement that the coup was a hastily improvised, eleventh-hour affair. It is misleading, therefore, to depict it as a deliberate gambit in the global strategic chess game. As we have seen, Moscow had not really taken the Afghan Communists seriously until 1976 and had not put together even a semblance of a unified Communist party until mid-1977. The party organization was still extremely thin in early 1978. Eventually, Moscow would no doubt have tried to put the Communists into power in one way or another, given the political vacuum that Daud had created by abolishing the monarchy and failing to replace it with another viable political system. As it happened, however, the Communists were forced to stage their coup long before they were organizationally prepared to govern the country. The Islamic revolt

Against this background, it is not surprising that some observers are asking how long Moscow will continue to back up its narrowly based client regime in the face of increasing militant Islamic rebels, risking embroilment in the same inhospitable terrain where British empire-builders suffered many unhappy adventures a century ago. One can only speculate, but it would be a mistake for the United States to underrate the elements of strength in the Soviet position, or to overrate the potential of the Islamic revolt against the Communist government.

A positive factor in the Soviet calculus is the existence of a capable, unified leadership dominated by Prime Minister Hafizullah Amin, who has steadily increased his control over day-to-day government and party operations and has emerged as the unchallenged heir apparent to President Taraki. Amin has successfully purged Parchamite elements, ending at least temporaily the chronic factionalism that plagued the party prior to the coup.

The Khalq leaders are relying heavily on economic appeals to the have-nots in the Afghan countryside. Acutely conscious that they started out with a limited organizational base, they are working furiously to make the most of their new power, tying in their party organizing efforts with land reform, anti-usury and educational programs.

Prior to the Communist takeover, the Afghan Islamic groups were not well organized politically and were never serious contenders for power. In recent months, the three Islamic factions have grown rapidly, aided by independently functioning guerrilla units led by former military officers. However, it remains to be seen whether the faction-ridden guerrillas, operating from base camps in Pakistan, can unite their forces and make effective continuing contact with the scattered rebel elements inside the country.

In assessing the potential of the Islamic revolt, one should avoid facile comparisons with recent events in Iran. Ayatohah Khomeini blended nationalist and religious symbolism, and even his specifically religious appeals had a nationalist flavor because Shiism has been historically fused with Iranian national identity. By contrast, the Pakistan-based Afghan guerrilla groups have not previously been identified with Afghan nationalist tradition. They represent, for the most part, the pan-Islamic movement linked with the Moslem Brotherhood.

In the eyes of many Afghans, one of the principal leaders of the Islamic exile groups, Sibghatullah Mojadidi, is tarred by the historic identification of the Mojadidi family with the 1924 overthrow of King Amanullah, the popular nationalist reformer. Historians are still arguing over the circumstances of Amanullah's downfall, but Afghan nationalists firmly believe that he was the victim of a British plot. The Communist regime seeks to depict itself as the heir to the Amanullah tradition and to discredit rebel leaders by branding them as "foreign lackeys" like those who collaborated with the British half a century ago. At the same time, the rebels are making a strong appeal of their own to nationalist feeling by pointing to the evergrowing role of Soviet advisers.

One of the imponderables in the Afghan situation is the possibility that the Islamic rebels will make common cause with non-Pathan separatist groups. There is a sharp ethnic and cultural cleavage in Afghanistan between the dominant Pathans, concentrated in the south, and the Tajik, Hazara and Uzbek minorities in the north. Historically, the non-Pathans have been largely excluded from political and economic power, and a significant underground separatist movement, seeking an autonous non-Pathan state in the north, has been active for more than 15 years. It was the principal non-Pathan separatist group, the Maoisttinged Setam-i-Milli (Oppressed Nation Movement), that appears to have been responsible for the abduction of U.S. Ambassador Adolp Dubs. The risks of involvement

In my view, Afghanistan is in for a protracted, slow-burning civil war in which the outcome may not be clear for some time. Faced with such an uncertain prospect, the United States would be wise to maintain a hands-off policy, seeking to keep the onus on Moscow for any expansion of hostilities between Kabul and its neighbors. The Soviet press has repeatly charged that Washington is already aiding the rebels, naming CIA agents who are allegedly assisting guerrilla operations. There is no independent evidence to support these charges, which have been strenuously denied by the United States.

Pakistan, however, has permitted guerrilla base camps to be established on its territory, and right-wing Islamic groups in Pakistan, emboldened by the hanging of former Prime Minister Zulfiqar Ali Bhutto, are pushing Pakistan's military ruler, General Zia Ul-Haq, to step up support of the rebels. Mojadidi, the rebel leader, told a recent American visitor that Saudi Arabian aids is on the way. Pressures for U.S. help are steadily growing, but American involvement would be extremely risky because Moscow could easily retaliate, unleashing guerrillas of its own in the politically vulnerable border areas of Pakistan and Iran.

One of the long-term dangers posed by a Communist Afghanistan lies in the volatile separatist movements among the 19 million Baluch and Pathan tribesmen whose homelands overlap the borders of Pakistan, Afghanistan and Iran. Increasingly restless, these tribal groups are divided between militant separatists who are looking for outside help and embattled moderates who are willing to forego independence if Pakistan and Iran will give them greater autonomy and more development assistance. Should the Afghan rebels step up their offensive with outside help, threatening the survival of the Kabul regime, Moscow could well respond by providing large-scale backing to separatist forces.

Pakistani Baluch guerrillas could give particularly valuable help to Moscow and Kabul by attacking rebel base camps in Pakistan, most of which happen to be located in Baluchistan province. Should Iran also give sancturary to the Afghan rebels, the Iranian Baluch could play a similar role.

Moreover, a military showdown over Afghanistan could well lead Moscow to make a larger strategic decision: that the dismemberment of Pakistan and Iran si necessary for defense of Soviet interests in the region. This would pose a serious dilemma for the United States, since it is not a Communist Afghanistan, as such, that threatens American interests but the possible use of Afghanistan as a springboard for securing Soviet political and military beachheads in the neighboring Persian Gulf area. One need only consider the implications of an independent Baluchistan, embracing the Baluch areas in Iran, which would control 750 miles of the Arabian Sea coast reaching up to the eastern approaches of the Gulf.

In time, Moscow might well give help to the separatists anyway, depending on the stability of the Afghan regime and the overall pattern of events in Pakistan and Iran. Soviet idelogists have long emphasized that the Baluch and Pathans are separate nationalities and have given intermittent support to the separatist cause in earlier years. But it is not inevitable that Moscow will embark on such a perilous course, and it would be a reckless gamble to force the issue prematurely. Pakistan and Iran still have time to take the wind out of separatist sails by making meaningful concessions to moderate tribal leaders on the autonomy and development issues.

The Communist takeover in Kabul came about when it did, and in the way that it did, because the shah disturbed the tenous equilibrium that had existed in Afghanistan between the Soviet Union and the West for nearly three decades. In Iranian and American eyes, Tehran's offensive was merely designed to make Kabul more truly nonaligned, but it went far beyond that. Given its unusually long frontier with Afghanistan, the Soviet Union would clearly go to great lengths to prevent Kabul from moving once again toward a pro-western stance. If the Islamic rebels prove successful, they would eventually have to come to terms with Moscow, returning to a Soviet-tilted neutralism, or they would undoubtedly face continual Soviet pressures and Communist-fomented civil war.

The issue before the United States, put in its baldest terms, is whether to follow in the shah's footsteps. American policymakers will have to decide whether a chancy attempt to eject the Russians from Kabul is worth provoking a showdown that could result in an uncontrollable pattern of regional destabilization - a cycle of challenge and response in which prophecies of a Soviet expansion could all too easily become self-fulfilling.