"SHOWING THE FLAG" -- displaying military power for diplomatic purposes -- is a venerable and useful practice, and over the centuries it has taken a variety of forms. Once it was gunboats, and more recently, in Saudi Arabia, a flight of F15s by way of trying to calm some of the nerves set on edge by the revolution in Iran. We are now witnessing yet another variation -- you might call it "showing the secretary of defense." Pentagon chief Harold Brown has been touring the neighborhood of the Persian Gulf, offering 1) assurances that the United States would itself respond to an appeal by a Gulf friend facing an "external" threat; 2) consultations on American plans for expanding its facilities at Diego Garcia, increasing American deployments in the Indian Ocean, and so on; 3) pledges of military equipment and training to friendly oil-producing states (and to their friends in North Yemen and Sudan); and 4) general all-around hand-holding. His mission has been uphill all the way, but within its modest possibilities it seems to have been a success.

What are the relevant political contingencies in and around the Gulf? Against the very real prospect of internal unrest, the United States can only encourage wise home-grown security and development policies -- and try harder to reduce its dependence on oil from Gulf sources. Against the more speculative prospect of intervention by the Soviet Union or a Soviet client, Washington could conceivably play a more active and even an armed role. The Brown trip comes in directly here, since the likelihood of foreign intervention will surely be affected by the perception of what the American response would be.

Another range of troublesome contingencies is functional and has to do with oil: embargo, sabotage, blockade. Everyone knows, from 1973-1974, that the only way to deal with a sovereign state's decision to embargo its own oil is to deal with the politics of it. That is precisely what gives urgency to the administration's effort to achieve an Egyptian-Israeli peace. A treaty would give the Saudis the extra reason they seek to cooperate in their own interest with the United States in oil and security affairs. That is why Mr. Brown told the Saudis, who have been disappointingly laggard in supporting the Camp David process, that its success is "central to long-term stability": theirs. A treaty would also give the Egyptians the opening they seek to become a more or less open surrogate of the United States in tending to the stability of the whole region. There is also the possibility of some Israeli role.

Sabotage in the oil fields is, by being intrinsically internal, the concern of the oil states. But what about a blockade of the tankers at, say, the Straits of Hormuz? For years a leading argument made by and in behalf of the shah of Iran to justify setting him up as a regional superpower was his role in preventing a blockade of that needle's eye at the mouth of the Gulf. To cope with the lone frogman, there can be no protection beyond general vigilance and the Arabworld mystique, such as it is, of the sanctity of oil. Against conventional threats, the armed forces of the oil producers and, in certain circumstances, of the consumers could come into play. Contingency planners earn their keep, we presume, by continually updating their plans

In fact, this country's friends in the Mideast, and therefore the United States, must live with a measure of uncertainty that prudent policy can diminish but not bring to and end. We take this as the strongest possible argument for prudent policy.