With most eyes turned toward the unraveling of Lebanon, Ayatollah Khomeini's Iran has launched a new offensive in the Gulf war - a conflict potentially more perilous to Mideast stability in the long run than even the Lebanon crisis.
The mounting concern of Arab and Western diplomats is that Islamic Iran - beacon for a surging Shiite Muslim militancy in the region - is steadily managing to wear down the Iraqis and their pro-Western Arab backers in the conflict.
This fear has begun to outweigh the more widely publicized, short-term concern that renewed fighting might activate Iran's threat to close the Strait of Hormuz if the Iraqis - tightly squeezed financially by the war - rocketed Iranian oil installations. Much of the Mideast's oil exports move through that 25-mile channel at the bottom of the Gulf.
The perceived danger of a closure of the Strait of Hormuz increasingly involves possible political, rather than economic, repercussions. World markets are, at least for the time being, awash with oil. One recently published Western newspaper report adds that a worried Saudi Arabia has shipped out an emergency stockpile of some 50 million barrels of crude in recent months as a precaution against any sealing of the strait.
What unsettles Western diplomats, and pro-Western regimes in the Mideast, is that an Iranian closure of the Gulf might leave the United States embarrassingly powerless to respond. The Lebanon crisis has already had a similar effect. Even acrobatic public statements from Washington have not dented a widely held view that the US has suffered a serious policy defeat there - a ''boon to the radicals, and a defeat for all moderates in the region,'' one Jordanian official said.
Although the Soviets find themselves in rare, shared enmity with the Ayatollah, it is far from certain that Moscow would actively side with any Western response to an Iranian shutdown of the Gulf. Economically, after all, the Soviets would stand to benefit from such a move, since they're the world's No. 1 oil producer.
Politically, the picture is murkier. But at a time of leadership transition, and of tension with Washington, the Soviets might find it hard to resist deciding to engage in ''anti-imperialist'' propaganda sniping in any showdown between the Ayatollah and the outside world.
First reports of Iran's long-expected military drive Thursday, as so often during the drawn-out conflict, were contradictory. But reports from both Iran and Iraq confirmed fighting was under way. The offensive, or at least its first stage, came considerably farther north than the Iraqis had expected - leading to speculation Tehran was intent on splitting Iraq's forces on the countries' 700 -mile frontier.
The Iraqis said they had struck back and contained the Iranian force. Indeed, Iraq has managed to fight off several earlier Iranian bids to push decisively across the border. But Western officials, in recent weeks, have expressed concern over a sustained buildup of weaponry on the Iranian side, and a steady recovery of the striking power of a sophisticated Iranian Air Force built under the late Shah.
Whether this buildup is of a sufficient scale to break the military stalemate of the past 31/4 years is far from certain, especially since Iraq, too, is understood to have taken delivery of new French- and Soviet-made missiles. But what is evident is that, gradually, Iran has won a commanding edge in the economic side of the Gulf war, reviving healthy oil exports while Iraq's oil-based economy has increasingly weakened.
This combination of factors is feeding concern in the West, and among its Mideast allies, that even partial Iranian success in its latest offensive might amount to a major boost for Shiite militancy in the region. In Lebanon, this week's military gains by Shiite Muslims, albeit still of a less radical stripe, are seen as having a similar effect.
One specific ''threat'' seen by Iran's foes is of suicide bomb strikes on targets in neighboring Gulf states. Iran has denied charges of having masterminded earlier such strikes, but has said in effect it is pleased that they've taken place.
Also causing fears among Iran's neighbors is the fact that some have sizable Shiite populations - a majority in Iraq, for instance, and in the Saudis' island neighbor, Bahrain. Many Shiites also live in eastern Saudi Arabia.
The Iranians have made no secret of their claim to leadership of an ''Islamic revolution'' they see as extending far beyond their own borders.
The results, from Iran's point of view, have so far been mixed - one certain disappointment being the failure of Iraq's Shiites to rise up and topple their government.
But the campaign continues. Two weeks ago, Radio Tehran announced the inauguration of an 800-kilowatt transmitter capable of broadcasting foreign-language transmissions ''to the whole of Saudi Arabia.''