IN MID-FEBRUARY, while world attention was riveted on Libya and the threat of international terrorism, military forces of revolutionary Iran flung themselves across the Shatt al-Arab River -- the historic dividing line between Persians and Arabs.

This bold gamble could turn Iran into a superpower in the oil-rich Persian Gulf.

Iran has maintained troops on the Faw Peninsula at the extreme southeastern tip of Iraq for more than three months. If it remains, the consequences could be far more serious than the latest antics of Colonel Qadaffi or the scattered spasms of terrorist violence.

Consider the following scenario:

*Iranian forces break out of the Faw peninsula to the west, severing Iraq's southern oil pipeline and its access to Persian Gulf ports.

*At the same time, Iran besieges Iraqi positions near Basra -- the "southern capital" of Iraq -- threatening Baghdad's lines of communications with the south.

*Khomeini declares an "Islamic Republic" in the "liberated" territories and installs a provisional revolutionary government built on a foundation of Iraqi dissidents.

*Iran emerges as a major rival to Saudi Arabia in setting oil policy and exerts enhanced political and military weight in Middle East politics generally.

This scenario seems far-fetched only because it has not happened -- yet. But this script is a description of the basic elements of a plan that Iran's revolutionary leaders have been pursuing with conscious determination -- and considerable success -- over the past year. If we are surprised again by Iran, as we have been in the past, we have only ourselves to blame.

Iran's recent successes were the result of conscious decisions taken to reverse policies that had brought it to a costly dead-end in its war with Iraq.

In February 1985, the Iranian leadership decided that its strategy of throwing wave upon wave of largely untrained young men against Iraqi tanks, mines and gunships was not working. After two years, Iran had little to show for its efforts beyond the proliferating martyrs' cemeteries that had become such a prominent feature of the landscape of the Islamic Republic.

Iranian forces were not moving, the pain of the war had struck nearly every Iranian family and grumbling about the war had produced sporadic outbursts of anger in the streets of Tehran.

Iran responded by a shift in strategy. In February 1985, Akbar Hashemi-Rafsanjani, the speaker of the Majles and Khomeini's personal representative on the Supreme Defense Council, announced that it was now Iran's intent "to achieve victory with as few casualties as possible." During the following year Iran shifted away from human wave offensives and began to rely on small scale probing actions and guerrilla attacks to harry Iraqi forces.

Iran also began making secret preparation for a bold military strike. Throughout 1985, Rafsanjani and others made tantalizing references to the "final attack" and the "main operations still to come." When Iraq bombed Iranian civilian targets, Rafsanjani accused them of trying to goad Iran into a "premature attack." However, it was easy to dismiss these statements as nothing more than hyperbole born of frustration. Iranian leaders had for years talked about a "final offensive" that never came.

Iran's military forces had been so debilitated by the lack of spare parts for its American-made equipment that it was reduced to a "march past" instead of a "fly by" demonstration by its air force on key holidays, apparently unwilling to put its few remaining planes in the air for any reason except vital defense. Throughout 1985, Iraqi planes owned the skies over Iran, pummeling its cities, its oil facilities and foreign ships engaged in the critical oil trade that sustained Iran's civilian and war economy.

Despite this barrage, Iran's revolutionary guards secretly began practicing amphibious maneuvers, preparing themselves for the assault that came a year later.

Last February 9, Iran's forces crossed the Shatt al-Arab River under cover of a rain storm that neutralized Iraq's superiority in the air. A combined force of regular military, Revolutionary Guards and irregular volunteers (basij), supported by the navy, broached Iraqi defenses on the southern flank and occupied the Iraqi port city of Faw.

This brilliant feat of arms, which bore comparison with Anwar Sadat's surprise attack across the Suez Canal in 1973, was no fluke. It demonstrated convincingly that the Iranian leadership was no longer motivated solely by religious fervor. Instead, after more than five years of hard experience, it opted for military professionalism, thorough planning and patient preparation.

The Iranian selection of Faw was brilliant but risky. Faw is an oil pipeline terminal that was rendered inoperative in the early days of the war. As a result, it was not regarded as a prime military target and was less heavily defended than other sites. It was also surrounded by water or marshy lands on all sides, thereby hampering Iraq's ability to reinforce it. Nevertheless, amphibious landings in a hostile environment are among the most technically demanding of all military operations, and Iran had little previous experience in this highly specialized form of warfare.

That they were able to plan and carry out such an operation under the most difficult circumstances is persuasive evidence that Iran has recovered from the uncertainty, dissension and disorganization that characterized its military forces in the years after the revolution. Like its revolutionary predecessors in France, Russia and China, it has succeeded in bringing up younger military talent that is committed to the revolution, and defeated what appeared to be a superior military force. It has overcome seemingly insuperable technical problems despite widespread international opposition and sanctions.

Its remaining problems are immense -- lack of spare parts, lack of modern weaponry, and the problems of command that arise from separate and potentially competing structures (the regular military, the Revolutionary Guards Corps, and the basij). But on the basis of recent performance, one can only conclude that Iran's military will be a force to be dealt with in the region for some time to come.

The same conclusion applies to the political leadership. Iran's theocratic political structure is unique, even bizarre by Western standards. Still, it has shown a remarkable ability to manage chaos and to protect its interests effectively when its survival is at stake. Despite international sanctions, nearly constant political turmoil and a bitter and costly war, Iran has sustained its oil production, paid off its international debts, built a range of new institutions that are taking root, and reorganized its military.

Very few countries in the world could have persevered in the pursuit of a military strategy as Iran did from February 1985 to February 1986, absorbing immense punishment from its adversary while patiently preparing a clever, well executed attack. More than any other single fact, that achievement suggests that the reemergence of Iran as the superpower of the Gulf is a prospect that must be taken with the utmost seriousness.

The outlines of Iran's strategy for the immediate future are not difficult to discern.

Militarily, Iran has put its total effort into the attack on Faw. It has thrown as many as 10 bridges across the river despite Iraqi air strikes. It will be difficult to dislodge. Khomeini set the tone last month in a meeting with his military high command. He congratulated them on their "perfect planning," then ordered them to "continue your ruthless battle and give them no quarter!"

Iran is presently massing a large force to the north of Basra, where it presumably intends to open a second front while Iraq is engaged in efforts to repel the Faw invaders. Rafsanjani, who serves as Khomeini's principal spokesman on the war, has declared that the current offensives "are also aimed at the oil areas in Iraq," which may suggest that the broader objective is to sever Iraq's oil outlet in the south. He has also stated that "what happened in Faw will happen in Basra as well."

In addition, Iran is organizing a new headquarters for guerrilla training, whose purpose is to "expand partisan and guerrilla warfare deep inside Iraqi territory." Finally, Iran is conducting small-scale attacks and is providing military support to Kurdish forces in the mountainous northern border region near the oil fields at Kirkuk.

It is likely that Iran, exploiting its sense of momentum and its 3-to-1 superiority in manpower, will press hard on all fronts in the months to come. Although Iraq enjoys overwhelming technological superiority, its tactics have tended to be sluggish, over-centralized and even inept with respect to the performance of many pilots. Iraqi forces have, however, demonstrated a dogged determination in defense of their own territory that should not be underestimated.

Preparatory to establishing an Islamic provisional government in the south of Iraq, Iran has actively supported the so-called "Supreme Assembly of the Islamic Revolution in Iraq" (SAIRI) headed by Iraqi dissidents, the most prominent of whom is Hojjat ol-Eslam Mohammed Baqir al-Hakim, son of an Iraqi ayatollah. He recently announced that a new SAIRI office had been opened in Faw.

Iran's terms for a settlement of the war have changed very little from the earliest days. Rafsanjani restated them as follows before the recent offensive began: "We only demand putting the criminal aggressor Iraqi President Saddam Hussein on trial and punishing him, in addition to compensation for losses, and the liberation of the Iraqi people. If our demands are met, we will then have no ambitions on a single inch of Iraqi territory." (Some Iranian spokesmen have pegged compensation at $350 billion.)

It is extremely unlikely that these terms could or would be met by Iraq under any circumstances short of an unconditional surrender. That is an improbable outcome, but it is equally improbable that Iran will significantly reduce its terms short of a major military reversal or a change in its own leadership.

Thus, there seems to be a considerable chance that Iran will remain on Iraqi soil, perhaps eventually operating through a political proxy such as SAIRI, awaiting the collapse of the Ba'athist regime and its replacement with an Islamic government more to its liking. In the meantime, Iran will attempt to truncate Ba'athist rule by splitting the Shiite south away from Baghdad and the Sunni north, while attempting to exploit its new-found political and military weight in the region.

The wild card in this latest twist of fortune in the Iran-Iraq war is, of course, the Ayatollah Khomeini himself. He is 83, and his death would send political shock waves through Iran. However, given the unanimity of the political and military leadership on the recent offensive, it is unlikely that the war with Iraq would be the subject of major dispute within Iran in the short term, even in Khomeini's absence. Moreover, steps have been taken to preserve basic political continuity in the event of Khomeini's death, and it does not appear likely that a radically different political leadership would emerge, at least for some time.

Thus, barring a major military reversal, Iran seems firmly ensconced on the Arab side of the Shatt al-Arab River -- a political shift of historic proportions. It remains to be seen how the United States, the Soviet Union, Iraq and the Arab states of the region will react to this change in a region where the interests of all of them potentially collide. By Gary Sick; Gary Sick was the principal White House aide for Iran in the Carter administration and the author of "All Fall Down: America's Tragic Encounter With Iran."