PRESIDENT REAGAN'S remarks about the Persian Gulf situation last Friday were almost bellicose towards Iran. This must reflect the depth of his wounds over the arms-for-hostages fiasco. But his personal pique should not determine how we go about fulfilling our commitment to keeping the Gulf open.

When we think about American naval involvement in the Persian Gulf, we need to take into account three facts:

There is no way to predict whether the Iranians will challenge our protection of shipping.

If the Iranians do attack, there is some chance they will succeed, because in war, there are no 100 percent defenses.

In response to a successful Iranian attack, the United States would be forced to escalate the hostilities considerably.

Unfortunately, we have been reacting to events in the Gulf without defining where we may be heading. Adm. William J. Crowe Jr., the chairman of the Joint Chiefs of Staff, was right to warn Friday that we shouldn't become more deeply involved in the Gulf unless we're ready to stay the course.

I'm not suggesting that we should not shoulder responsibility for protecting shipping in the Gulf. What little credibility we have in the Middle East would be shattered if we walked away from what we have repeatedly declared to be a "vital national interest." What I do suggest is that we need to think through how we will react if we are attacked, and what the consequences will be.

We aren't commiting American power simply to defend 11 Kuwaiti ships flying American flags. If Iranian attacks begin to take a substantially higher toll on general shipping than they have in the past, we will be seen to have failed. Our task will be nothing short of ensuring a reasonably normal flow of non-Iranian shipping in and out of the Gulf.

We have two ways of defending shipping in the Gulf: riding shotgun for individual ships or convoys (which I will call "point defense") and attacking the source of the threat, the Iranian air force and navy, in their bases. Until the Iranians strike a first blow, we are, for all intents and purposes, limited to the point-defense option. We do not want to initiate a war with Iran.

The damage done to the USS Stark raises questions about the benefits of point defenses. The problems that the Stark's sister ships will face are clear. On one extreme, any ship is vulnerable if the attacker gets the first three shots. Modern missiles are lethal. Modern ships are not ringed with armor and must depend on their self-defense systems, and those may have only a few seconds in which to react. On the other extreme, any ship's self-defense system can be overwhelmed by a mass attack, perhaps 10 simultaneous missiles.

The Stark was close to the first extreme. Whether her captain and crew did the most they could to protect the ship will be determined by the naval investigation now taking place. Surely, though, the captain was at the disadvantage of being in that twilight zone between peace and war. A good bit of the Stark's handicap has been eliminated as we have moved closer to a war footing.

The risk at the other extreme -- of mass attacks -- is not high, primarily because we do not credit the Iranian air force and navy with the capability for large, coordinated attacks.

We are in between. The issue is whether the Navy can improve the chances that our point defenses will be successful. Under the plan announced Friday, the United States will increase its Persian Gulf task force by three destroyer-type ships, including a more sophisticated missile cruiser. Whether that will be suffcient remains to be seen. But clearly, the new deployment will improve the prospects for point defense.

We could also attempt to provide air cover during daylight hours, on the assumption that the Iranian capabilities for night attack are low. The president's plan includes a provision for at least partial air cover. But unless the Navy is willing to bring its aircraft carriers right into the Gulf or the Air Force is able to obtain the use of air bases on land, it would take a prodigious and very expensive effort for carriers outside the Gulf to maintain air cover over shipping inside the Gulf.

The geography is forbidding. The carrier would probably be 150-200 miles outside the Gulf, which is itself some 500 miles long. That is a lot of territory to cover.

There is also a problem of geometry. Protected aircraft cannot just be anywhere over the Gulf. They must be able to race to a ship under threat faster than an Iranian aircraft can get from its base to a point 30-40 miles from the ship and launch a missile. From one Iranian air base it is only 120 miles across the Gulf and, so, our aircraft would have to be almost on top of the ships they were protecting. Land bases are preferable, but they may be ruled out by the local politics. Even from land bases it would be an expensive operation.

Iranian air attacks are not the only threat. The Iranians have missile boats that could dash out into the Gulf; they have Chinese "Silkworm" missiles, which could be mounted on land near the Straits of Hormuz to fire at passing ships; and they have mines that could be placed in the Straits.

These threats are probably manageable. Mining a strait that is 30 miles wide is a large undertaking, which we should be able to detect and stop; the Silkworm is a relatively large missile that our Air Force AWACS surveillance aircraft and our ships should find easier to detect than air-launched missiles; and the missile boats take a lot longer than aircraft to get near their targets and should be detected.

It will cost something to defend against each of these added threats, though. To minimize the costs of point defense, the president's plan is to aggregate tankers in small convoys. That, though, has drawbacks. When a convoy arrives at the oil terminal, there will likely be too many ships to load all at once. The resulting delay will mean that the tankers will remain vulnerable inside the Gulf for longer periods than if they proceed individually.

The bottom line is that the U.S. Navy, at a cost, can increase the probability of successful defense. But the probability will never be 100 percent. There is just too much room for innovative tactics by the enemy to surprise us, poor reflexes on our part or just plain luck. If the Iranians opt to run against high odds, they may just damage or sink another U.S. warship.

What options would the president have if the Iranians did take us on and do serious damage? The prospects of a third warship damaged or sunk would loom as a political disaster at home. It would also seriously undermine confidence in whether the United States can protect its vital interests in the Gulf. At that point, I believe the president could no longer rely on point defenses.

He would have to shift to the tactic of attacking the threat before it was launched. That would mean using our aircraft from several carriers to attack Iranian air and naval bases. By eliminating as many of the Iranian aircraft, ships and missile installations as possible, we could reduce the threat appreciably. Such attacks, of course, would be a major escalation, but they may be necessary. The long-term political consequences for our relationship with Iran could be significant.

Whether Iran will force us down this track by challenging us to combat, I would not want to predict. We Americans have been abysmally poor at reading the Iranian mentality over the last 10 years or so. There is, though, one action the Iranians could take at small expense that would be very tempting. They could force us to stay on this costly alert by making threatening feints. Even if they went no further, that would keep tensions high and run the risk of inadvertent hostilities.

In short, we may be compelled to initiate broad hostilities against Iran, or we may simply be drawn be drawn into such a conflict. We ought to understand this danger as we begin our new role in the Gulf. All too often, presidents and their advisers embark on military actions in the hope that the first step they take will solve the problem. Often, it does not. In this instance, the first step of providing defense for shipping may do the job, but we would be foolish to count on it.

The Iranians must also understand that we will be as aggressive as necessary to fulfill our role. Undoubtedly, the president's strong remarks Friday were intended for this purpose. We need, though, to be sure we do not let ourselves be driven by anger or political pressures. We shouldn't be eager for hostilities with Iran. We have a strategic interest in reestablishing ties to Iran one day, and we do not want to leave the field to the Soviets. Initiating large-scale hostilities with Iran would push the day of reconciliation further and further off.

Another reason to avoid hostilities is the anomalous position of the Soviet Union in the Persian Gulf today. Like us, the Soviets are committed to the defense of some of the tankers in the Gulf. They face some of the same dilemmas about what to do if their forces are attacked. Thus, we and the Soviets have a coincidence of interests in bringing the Iran-Iraq war to a halt as soon as possible.

That will mean, though, that the Soviets are going to claim a place at the conference table, something we have attempted to avoid for years. We have backed ourselves into this corner with the mishandling of our dealings with Iran over the past several years. Now, the more deeply we become involved in hostilities with Iran, the greater the Soviet voice in the eventual Persian Gulf settlement will become.

This, then, is no time to let the residual resentment we have for Iran -- stemming from the 444-day hostage crisis and the recent embarrassment of the arms-for-hostages dealings -- dominate our responses. We must be willing to escalate hostilities with Iran if necessary to fulfill our mission, but we should do so only as a result of cool judgments and with a recognition that there will be serious consequences.

Stansfield Turner, a retired Navy admiral and former director of Central Intelligence, is working on a new book, "Terrorism and Democracy."