When the Reagan administration has embarked on a new foreign policy venture, the president often has signed a document known as a "National Security Decision Directive" outlining the strategy behind the policy.

But this bit of paperwork was not prepared in the case of America's recent military buildup in the Persian Gulf. The reason, officials say, was simple: Nobody thought a formal new interagency study of our gulf policy was necessary. The decision to reflag and escort Kuwaiti tankers had broad support within the administration and didn't require any formal decision directive from the president.

The lack of a new NSDD illustrates some of the best and worst aspects of U.S. policy in the Persian Gulf. And it shows how the United States has moved so quickly -- and with so little debate -- into a military confrontation with Iran that involves dozens of U.S. warships and thousands of sailors.

Administration officials cite several factors that led to swift approval of the new gulf policy:It was regarded as an extension of existing U.S. commitments in the gulf, rather than as a new departure. The pledge to defend moderate Arab regimes in the gulf dates from the Carter administration and has been reaffirmed repeatedly by President Reagan. Indeed, one official said last week that the basic rationale for current U.S. policy in the gulf is set forth in several NSDDs that are more than three years old. No new NSDDs on gulf policy have been prepared since then, he said. It had strong support from both the Pentagon and the State Department, which have often been at loggerheads in the past. Given the military's usual wariness about the use of military force, its support for the gulf mission has been crucial. "This is the Pentagon's baby," said one former official. "Bureaucratically in this town, if the Pentagon is in favor, that's it." It was seen as a tough-minded response to threats posed by the Reagan administration's two favorite enemies: the Iranians and the Soviets. Adding to the pressure for a strong American response to Kuwait's request for protection was the fact that it came after the embarrassing Iran-contra affair and that it came early in the tenure of a new national security adviser, Frank C. Carlucci, who wanted to show that he was in charge. These factors combined to produce a policy that was shaped largely by a desire to demonstrate American "credibility" in the gulf region.

"In contrast to Lebanon, there has been very little interagency quarreling about the overall direction of our strategy in the gulf," said Geoffrey Kemp, a former NSC aide who supervised gulf policy during the administration's first term. "Basically, the Pentagon and the State Department have always been in agreement on this. That's why there was so little discussion of it, why Congress was so indifferent, and why there aren't little pieces of paper around town outlining our strategy."

Despite the initial consensus about policy, some mid-level officials have privately begun to express skepticism. What worries them is that in the rush to respond decisively to events in the gulf, the administration may have overlooked some of the strategic difficulties that may result from the show of force.

The chief concern is that the tanker-escort policy could draw the United States into a shooting war with Iran. Several officials say that if Iran attacks American ships, the United States would probably retaliate by bombing military targets in Iran -- setting off a conflict that would unite Iranians against the United States and could push them into the arms of the Soviets. In this conflict, the religiously motivated Iranians might have greater staying power than the United States.

Already, critics warn, there is a danger that the United States is taking over from Iraq the risky task of trying to contain the Iranian revolution. This commitment is implicit in a summary of U.S. policy offered by one of its architects. Our policy, this official said, "is an attempt to contain the Iranian revolution until it calms down."

That could take awhile. For this reason, some analysts worry about the open-ended nature of the U.S. commitment to escort Kuwaiti tankers. Asked when the Navy convoys might end, a senior administration official said: "If tensions cool down in the gulf, we can stop escorting tankers." Critics argue that this loose formulation allows Iran to dictate the terms of the conflict.

Another problem, cited by several critics, is that the tanker-escort scheme may not advance the American goal of pressuring Iran to accept a negotiated settlement of the war, now in its seventh year. Instead, the U.S. commitment to keep gulf sea lanes open may actually help Iran, which exports its oil by sea, rather than Iraq, which does not.

Critics also question whether America's gulf policy is sustainable politically. They note that the administration is, in effect, allying with the same moderate Arab states -- Saudi Arabia and Kuwait -- that Congress has balked at supplying with weapons, largely because such weapons sales are opposed by Israel. Some skeptical officials fear that the gulf policy is built on the same shaky political foundation as was the commitment of American troops in Lebanon.

But senior administration officials who crafted the policy reject such criticisms. The alternative to our policy of protecting Kuwaiti and Saudi shipping, one official says, is "the policy of allowing the Iranians and Soviets to have dominance in the gulf. If the body politic decides they want that, that's their choice. We think that's a big mistake."

The rationale for supporting the gulf states was spelled out in several NSDDs signed in mid-1984, when the Iranians began attacking Saudi and Kuwaiti shipping, the senior official said. A May 1984 NSDD titled "Improving U.S. Posture to Respond to Developments in the Iran-Iraq War" discussed actions that might be taken to help the gulf states. And an NSDD signed in June 1984 pledged that it would be "U.S. policy to undertake whatever measures are necessary to keep the Strait of Hormuz open."

The premise of these 1984 NSDDs was that the United States was prepared to use military force, if necessary, to counter Iran in the gulf war. The documents discussed such issues as access to local bases in Saudi Arabia and elsewhere for U.S. forces that might be deployed to the gulf; plans for political and military consultation with key gulf states, and measures to enhance the readiness of U.S. forces in the region.

The official cited these three-year-old documents as evidence of the continuity of policy. Rather than drafting a new NSDD dealing with the reflagging of Kuwaiti tankers, the official said, the administration set forth its new Persian Gulf strategy in two brief public statements by Reagan, issued Jan. 23 and Feb. 25, and in a 30-page report to Congress issued June 15 by Secretary of Defense Caspar W. Weinberger.

The steps that led to the reflagging decision -- and the massive naval buildup in the gulf -- are outlined in Weinberger's June 15 report to Congress. The report makes clear that the administration was drawn into its Persian Gulf role largely by fears about growing Soviet influence in the region.

The Kuwaitis first queried the U.S. Coast Guard about procedures for reflagging tankers last Dec. 10. But the reflagging issue did not receive serious top-level attention until after the United States learned on Jan. 13 that Kuwait had received an offer of naval protection from the Soviet Union. From that point on, U.S. policy developed quickly and, it seems, with little controversy.

The administration's worries increased in February, when the Soviets offered to reflag and escort five Kuwaiti tankers and the Iranians tested Silkworm antiship missiles. In his Feb. 25 statement, Reagan warned: "We remain strongly committed to supporting the self-defense of our friends in the region, and recently moved naval forces in the Persian Gulf to underpin that commitment."

By the first week of March, the president had approved a recommendation that the United States offer to protect Kuwaiti ships, and on March 10, the Kuwaitis indicated they would accept the offer. The administration was still working out the details on May 17, when the accidental Iraqi attack on the USS Stark brought home to Americans the dangers of becoming involved in the gulf war.