One year after the loss of the U.S. frigate Stark and 37 of its crewmen in an Iraqi missile attack, the U.S. military buildup in the Persian Gulf has bolstered American credibility in the region and contributed to closer strategic cooperation between the United States and the conservative Arab nations along the vital waterway.

But due to what some critics say is still a confusion of aims and a consistent misreading of Iranian actions and reactions, the stronger U.S. presence has not yet achieved the more vital purpose of hastening an end to the nearly eight-year-old Iran-Iraq War.Moreover, positive U.S. achievements have been purchased at a price for which the 46 American lives lost in the gulf so far may only be the down payment. Begun with great fanfare and flag-waving one year ago, the U.S. military commitment in the gulf is now open-ended, deepening by the day and impossible to reverse except at the cost of all that it has accomplished thus far.

This assessment, generally positive but tinged with much anxiety, reflects the views of more than a dozen Arab officials and Western diplomats interviewed in the gulf over the past week. Their consensus can be summed up as follows:

So far so good, but the worst - and with it, the real test of America's determination - may be yet to come.

Even by the standards of destruction made possible by today's conventional weapons, the Persian Gulf War has been a particularly gruesome conflict - a macabre marriage of World War I-style trench warfare and attacks by modern missiles and chemical weapons that has claimed more than 1 million lives with, in the end, little territorial gain or loss by either side.

It is the longest conventional, or non-guerrilla, conflict of the 20th century, and it has been going on for so long that, to all save Iran, its origins seem almost irrelevant now. Iraq started it, but Iran refuses to end it.

And yet even in this seemingly interminable conflict, whose outcome is as difficult to foresee as its beginning is to reconstruct, a few dates stand out.

One such date was May 17, 1987, the day that an Exocet missile fired by an Iraqi plane slammed into the side of the Stark while the guided-missile frigate was on a routine patrol in the central gulf northeast of Bahrain, resulting in the loss of 37 American lives.

It was all a terrible mistake, the Iraqis said afterward, and the United States accepted that explanation. Yet it was also a fateful mistake, one that, in the opinion of most analysts, helped the beleaguered Iraqis to realize their long-held ambition of internationalizing the gulf war.

For, while the United States had already agreed to Kuwait's request for naval escorts to protect its oil tankers from Iranian attacks, it was the Stark incident that, by dramatizing the dangers American sailors now had to face, was largely responsible for the deployment of U.S. forces in the gulf on a massive scale.

At first, the seeming haste and confusion with which the U.S. deployment was subsequently launched led the timid Arab states facing Iran along the western shore of the gulf to view it with grave misgivings.

They feared that internationalizing the war would lead to its uncontrolled expansion. They feared that Washington was laboring under two illusions: that it could accomplish its mission in the gulf fairly quickly and without serious cost, and that Iran would shy away from a confrontation with the United States and eventually conclude that it had no choice but to make peace with Iraq. Most of all, they feared that, having barnstormed into the gulf amid a lot of media hoopla, the United States would "cut and run" when reality sank in and more American lives were lost.

That has not happened.

It is true that, while the U.S. Navy convoys have protected Kuwait's tanker fleet, the American presence has not as yet contributed significantly to the larger aim of making the gulf safe for international shipping.

Yet, in dealing in a measured but firm way with subsequent Iranian provocations, in demonstrating its determination to keep the gulf's narrow sea lanes free of Iranian mines and, most of all, in maintaining its resolve even in the face of nine more American deaths, the United States "has finally convinced its gulf allies that it means what it says," one diplomat said.

The effects of this turnabout are hard to quantify, even though they are plainly apparent. The U.S. presence, for instance, has helped to embolden the gulf states into taking a more confrontational stand of their own against non-Arab Iran, thus increasing Iran's diplomatic isolation. Saudi Arabia portrayed its cutoff of diplomatic relations with Iran last month as an independent decision, yet most analysts doubt that the Saudis would have made the move had the United States not previously proven its commitment to the security of the Saudis state and other allies in the gulf.

Thus, despite a shaky start, the U.S. deployment has evolved over the past year into what Ambassador Sam Zakem, the U.S. envoy to Bahrain, describes as a "glowing success . . . the most successful example, in fact, of American policy anywhere in the world today."

Recent interviews with other Arab and Western officials up and down the gulf revealed a similar, if less effusive, consensus that the United States has managed its role in the region intelligently and successfully.

This, at any rate, is one way of looking at it. But there is also another, more pessimistic view that recognizes that the United States has consistently failed to understand Iran's "Through the Looking Glass" logic and thus has been taken by surprise on a number of occasions by the Iranians' seemingly irrational behavior.

This was clear on April 18 - another significant date in the gulf war - when U.S. warships destroyed Iran's Sassi and Sirri oil platforms in retaliation for the Iranian-laid mine that injured 10 sailors and nearly sank the U.S. frigate Samuel B. Roberts in the central gulf four days earlier.

It was apparent, diplomats say, that the United States did not anticipate Iran's response. The U.S. Navy had destroyed another Iranian oil platform last October in retaliation for an Iranian attack on a U.S.-flagged Kuwaiti tanker and, apart from a lot of threatening rhetoric, there was no immediate Iranian response.

This time, however, the Iranian navy engaged the U.S. fleet. In the ensuing fighting, six Iranian vessels, including two frigates, were either sunk or badly damaged. One American helicopter, along with its two-man Marine crew, was lost.

Until April 18, Iran's warships had steered clear of the U.S. Navy. The use of regular naval forces this time suggested an element of calculation that the United States clearly had not counted on, diplomats said.

It is possible, though unlikely, that the Iranians thought they could really deal a damaging blow to the U.S. fleet. Far more likely, however, diplomats said, is that the attack was ordered to distract attention at home from the more damaging defeat Iran suffered a day earlier, when Iraqi forces recaptured the Faw Peninsula, lost to the Iranians two years earlier.

"They will choose their own time and their own place, but one thing's for sure: The Iranians will try to find a way of retaliating for April 18," said a Dubai-based shipping official who has been following the war since it started. "You Yanks have done a pretty good job so far, but if you want to win this game, you'll have to do one thing better.

"You'll have to learn to predict the unpredictable."