AFTER 18 MONTHS of bloody warfare between Iran and Iraq, the most important thing about that war is what has not happened. Only a few years ago, the Persian Gulf was widely viewed as the world's most explosive powder keg, where just one strategically placed sunken freighter supposedly would block a major part of global oil exports and threaten a cataclysmic Soviet-U.S. showdown.

Jimmy Carter gave his name to a doctrine that said the United States would defend its interests in the Gulf, by force if necessary. The Russians replied with warnings that they would match American military intervention, and cited a nearly forgotten 1921 treaty with Iran.

But faced with a genuine crisis -- the war -- the two superpowers have been reduced to policies falling somewhere between benign neglect and frustrated impotence.

In fact, the war has not significantly disrupted world oil trade. It has not dragged the Soviets or Americans significantly deeper into the region. The conflict has not overtly destablized the neighboring, fragile states in the Gulf by spreading the brand of Islamic fundamentalism championed by Iran's ayatollahs.

Initiated as an overconfident gamble by Iraq's Saddam Hussein, who had been assured by exiled Iranian generals and politicans that Ayatollah Ruhollah Khomeini's government and its newly purged army would quickly crumble, the war has now become a contest between two highly individualized regimes that apparently will not stop fighting until either Saddam Hussein or Khomeini leaves the scene.

Normal military, economic and diplomatic pressures that might bring an end to such a war are dissipated by the fanatical zeal that each leader has been able to impose on his nation. The mutual determination to outlast the other has helped reduce the ability of any outside power to work to end the fighting.

But it is also clear that the Kremlin and the White House have separately but perhaps in parallel reduced the priority they had assigned to turmoil in the Gulf just three years ago. The visions of world-shaking turmoil created by the fall of the shah and subsequent outburst of Islamic revolutionary fervor and by the Soviet invasion of Afghanistan have palpably subsided.

Of course one other thing has not happened -- the war has not ended. The dangers of the conflict spilling over have not disappeared. Iran continues to press its offensive inside Iraq, and an Iranian breakthrough would probably create panic among conservative U. S. allies like Saudi Arabia and Jordan. That, in turn, could bring a higher American involvement and an unpredictable Soviet response.

A search for some of the principal implications of the Gulf war for superpower rivalry was launched last month at Ditchley Park in England, at a conference of experts who work both on the the Middle East and on Soviet-American relations. Participants reached no sweeping conclusions, but the discussion did suggest some of the lessons of this unexpected episode.

In limited ways superpower interests have been affected by the war, particularly through a net loss of Soviet influence in the short term in both Iran and Iraq. Both have rejected clumsy Soviet efforts to keep a foot in each camp.

But the verbal thunderbolts that Carter and Brezhnev hurled at each other over the Persian Gulf three years ago have stopped echoing. Today the two superpowers are concentrating their efforts in a battle for public opinion and military advantage in Europe. The Kremlin in particular appears to have shifted its priority and to be wary of expending too much energy and attention on other areas when the stakes in the battle over missile deployment in Europe are so large.

For America, the oil glut has enabled the Reagan administration to stand by relatively calmly as the two anti-American Gulf regimes ravage each other. U.S. strategic concerns have returned, at least temporarily, to the pre-1973 norm; the chief American interest is that Gulf oil keep flowing to its NATO allies and Japan, rather than directly to the U.S. economy.

Until recently, a tacit "balance of terror" seems to have been operating along the Gulf to permit a certain level of shipping to continue. While Iraq has attacked some Iranian oil facilities, it has not conducted the kind of intensive inderdiction effort that qualified experts feel it could undertake, and tankers carrying Iranian oil continue to move through the Gulf.

And after a warning air raid into Kuwait that seemed intended to establish a point about mutual vulnerability, Iran has not struck at the Kuwaiti and Saudi oil fields that produce the tens of billions of dollars that have underwritten the Iraqi war effort.

Like the threat to international oil supplies, the threat of rapidly spreading "Khomeinism" has also declined as this war has worn on. The chaos, intolerance and repression that have accompanied the founding of the Islamic Republic in Iran have significantly lessened the possible appeal that the Iranian brand of Moslem fundamentalism might have had for Arab Moslems across the Middle East. Arab participants at the Ditchley conference repeatedly challenged what they saw as an overemphasis by their Western colleagues on the prospects for the war spreading Islamic fundamentalism throughout the region.

If the war created theoretical opportunities for the Soviets, in fact they have been unable to exploit them. An initial Soviet tilt in favor of Iran, evidenced by arms shipments to Tehran through Soviet client states, apparently did not win over the fiercely anti-communist mullahs, who were deeply upset by the occupation of Afghanistan. In recent months, after attempting to keep footholds in both camps, the Soviets have resumed arms supplies to Iraq and diminished their involvement in Iran. Khomeini's government quickly responded with a crackdown on the communist Tudeh party inside Iran and resumed bitter polemics against Moscow.

In one view that has currency inside the Soviet leadership, the war has been costly politically for the Soviets, with both Iran and Iraq moving steadily to the right in domestic politics during the course of the war. There are growing signs that Soviet disappointment with Tehran has led Moscow to turn its attention to pushing for renewed influence in Baghdad and a Syrian-Iraqi rapprochment that would give the Russians a way back into the Middle East. Success in bringing about the rapprochment would signal a decisive shift in favor of Iraq in the war.

If the superpowers have exercised relative restraint during the course of the war, there is little question that their willingness to pour massive amounts of sophisticated weaponry into both countries in the 1970s provided the stockpile that has kept the battle going for so long.

And a tier of intermediate arms suppliers operating through the black market and existing procurement networks outside of direct superpower control has emerged during this conflict, which may set patterns for future regional wars. Emergency French shipments of arms saved Saddam Hussein from defeat last fall during the most intensive Iranian push. North Korea and Czechoslavakia have profited handsomely from their sales, and Iran was able to call on the military procurement network that had existed between the shah and Israel to get small amounts of war equipment at crucial moments.

Both Washington and Moscow rationalized their willingness to sell the most advanced conventional weapons in their inventories to Tehran and Baghdad on the grounds that the sales would enable them to maintain influence in that potentially explosive region. Now that the explosion has come, the influence has vanished.