By placing its troops in direct combat and garrison roles to install a tractable new regime in Afghanistan, the Soviet Union has come closer to achieving an objective that eluded the czars for more than a century: control of a landlocked central Asian area that eventually could yield to Moscow direct access to warm-water ports. This is a strategic goal of immense importance.

In a broad sense, the Soviet military thrust is a shrewd move on the world geopolitical chess board of the sort the Brezhnev leadership has made in various forms in recent years in Angola, Ethiopia and Cambodia. Moscow's intervention seems to guarantee the swift conversion of an innocuous buffer state into a Kremlin puppet and potential staging area for direct leverage deep inside Central Asia.

Iran, Pakistan and India woke up to a changed world Friday after the coup Thursday in Kabul. The intervention also is significant for China, the Soviet Union's most bitter adversary, which has expansionist aims of its own in the area.

The angry and worried Washington reaction, which President Carter made plain in his luncheon talk with reporters today at the White House, underscored the fundamental strategic change Moscow's move implies and the strength of Soviet military power. At a time when the distant United States is grappling with ways to reinforce its influence throughout the Persian Gulf and South Asia, the Soviets simply marched in against virtually no organized opposition and took control. The operation reflects continued Kremlin calculations that it can seek detente with the West while also seizing any opportunity to expand its power. Whether it can consolidate this gain and make more from its new salient depends on various internal and external factors, but the historic goal is clear.

The Arabian Sea port towns of Chah Bahar in Iran and Gwadar in Pakistan lie about 300 miles south of the Afghan border, in a region dominated by Baluchi tribesmen disaffected from both Tehran and Islamabad. Baluchistan, convulsed by a breakway rebellion from 1973 to 1977, has always been a tempting target for a Soviet gateway to the sea.

Strategic international oil sea lanes to the Persian Gulf lie south of the Baluchistan coast. But Moscow first must deal with complex tactical and strategic problems within Afghanistan, which could take years to solve and could themselves damage Soviet relations with Moslem governments and peoples from the Persian Gulf to the South China Sea.

The success of the Soviet gamble, which began with the relatively easy coup against President Hafizullah Amin, depends on the ability of new leader Babrak Karmal -- to quell the Moslem insurrection that has ripped Afganistan since pro-Marxists came to power in a 1978 coup.

The fact that Moscow mounted the military overthrow underscores the consensus among foreign observers here that the Soviets are intent upon finding a political settlement allowing peace between the pro-Marxists and the tribal Moslem insurgents. Babrak, sheltered and tutored by Moscow or its East Block allies this year going underground to escape a purge by Amin, has called for peace with the Moslem tribesmen. However, Moscow is in the awkward position of using its troops to quell rebellious Afghan Moslems while proclaiming it supports Moslem nationalist aspirations elsewhere in the region.

To minimize the impact of this, the Soviets are certain to clamp tight restrictions on outside access to Afghanistan to lessen internal awareness of what is going on. But if Moscow expands its combat role, much will be learned abroad from foreign diplomats, Western intelligence listening devices and from thousands of new refugees streaming into Pakistan to join the estimated 350,000 Afghans already there.

Official Soviet media have offered only terse confirmation of Russian combat troops in Kabul. Today, the official Tass news agency reported a Kabul radio address by Babrak and highlighted his assurances to "the worthy Moslems of Afghanistan, Sunni and Shiite, ulemas and clergy."

But Moslem countries elsewhere have already acted with alarm to the intervention, providing a foretaste of longer-term reaction to the Middle-East and South Asia. The Kuwaiti government has officially protested and newspapers in Saudi Arabia and Iraq have denounced the Soviet role.

Iran's Ayatollah Ruhollah Khomeini also has responded negatively, and it is here that the Soviets may stand to lose the most ground in the immediate future. Khomeini earlier had attacked Kabul for warring on Moslems. Tehran has always been particularly sensitive to its problems with the Baluchi separatists. Khomeini, who has sought to solve his severe internal problems by uniting Iranians behind his handling of the U.S. Embassy crisis, is unlikely to soften his view. Indeed, the Soviet intervention calls up memories of the Stalin-era seizure of northwest Iran during the World War II Anglo-Soviet operation to secure its oil fields from the Germans. The Kremlin later unsuccessfully attempted to carve out an "independent" Azerbaijani state there.

Elsewhere in South Asia, the Soviet move will further strain relations with Pakistan, which already stands accused by the Communist Party paper Pravda of supporting the Afghan Moslems. The coup came just before Islamabad was to send a diplomatic mission to Kabul to seek ways of easing mounting ill-will over the presence of Afghan refugees in Pakistan's border areas. The camps have been centers of some insurgent activity, despite strict Pakistani measures to suppress inflammatory public pronouncements by anti-Marxist leaders there.

While it has exacerbated relations with many Third World countries, Moscow's presence brings with it in the long run enormous new pressures for accomodation with the Soviets by those capitals. In addition, some veteran Asian diplomats here believe that the Kremlin's action also has -- perversely -- given it a significant and potentially long-term psychological gain over the United States.

These sources argue that Asian and Moslem leaders, accustomed to wielding power to govern societies prone to mass violence and turmoil, have gained new respect for Moscow's naked use of force to stake out its vital interests. Although they back Washington's use of restraint in handling its Tehran embassy crisis, these diplomats say the United States is the loser in any comparison by some Asian capitals with Moscow's Afghan intervention.