IN 1979, when the Soviets invaded Afghanistan, what was alarming about the act -- in addition to the brutal aggression against the Afghans -- was that Soviet power had moved that much closer to the Gulf. That region was recognized as intrinsically ''strategic'' and of ''vital'' interest to the United States. Recall how often a case has been made against American involvement in other areas on grounds that these weren't the Gulf, weren't unequivocally strategic -- a designation summoning up a whole higher set of stakes, costs and risks.

We cite this bit of background by way of observing that President Reagan struck the right, geopolitical note in his Gulf references Monday night. He has been accused of beefing up the American naval presence there 1) in response to the Soviets' strengthening theirs and 2) to cover his embarrassment over his arms-for-hostages dealings with Iran. But these considerations are real. It would have been a geopolitical revolution for the United States suddenly to concede to the Kremlin a premier patron's role in the Gulf. It would have been a dereliction for the government, being faced with the general Arab conviction that it had enlisted on Iran's side of the war with Iraq, to ignore the requirement to make a gesture for the Arab cause.

A burden falls on President Reagan to show that the steps he proposes in the Gulf serve, and serve efficiently and prudently, the large and legitimate purpose of preserving an American position in a vital region. To judge by the misgivings still being expressed in Congress and elsewhere, he has a way to go to make this demonstration, and may yet have to scale down some of his plans. Most conspicuously, he failed to make the case for engaging Saudi Arabia, whose quiet cooperation is critical to the American military mission; the lapse let a myopic Israel lobby kill the proposed sale of Maverick missiles to the Saudis. He has also failed to build a constituency for the diplomacy he has undertaken at the United Nations Security Council.

But a burden also falls on critics to give a fair hearing to the president's strategic case, and not to terminate their contemplation of the issue with a recital of the risks. And if they do give a hearing and determine that his case is valid, they must follow through -- by voting for the Maverick sale when it comes up again, for instance, and by supporting efforts to negotiate an end to the Iran-Iraq war.