Only last year Defense Secretary Harold Brown was saying that "we are as yet unsure of the utility of U.S. military power in Persian Gulf contingencies," but now he has detected "some sort of great watershed or hinge of history" and wants to spend an extra $10 billion or so over the next five years to project American military power into . . . the Persian Gulf.

This has happened just as the Iran crisis has painfully underlined the wisdom of his earlier insight. Moreover, no one -- certainly not Brown -- has attempted to spell out for the public the sorts of future Persian Gulf contingencies in which the forces he contemplates could be usefully employed.

Harold Brown is smart and all that, but he appears to me to have gone off the deep end. The Iran crisis has jostled all of us, and the administration has taken the occasion to do some useful catch-up work on the military budget. In the process, however, it's giving the impression of being rolled over by the bulldozer that the generals and admirals had unchocked and ready for just such a moment of civilian political vulnerability. The plans for rapid deployment forces are the most conspicuous result.

What is the RDF? It doesn't exist. It hasn't yet been planned. The planning group meant to plan it has not been set up. "The arrangement that we plan," Brown said last Friday, "is to have a joint task force to do the planning for various contingency uses of the rapid deployment forces and the rapid deployment forces, as you heard, could comprise anywhere from battalion to an Army corps in size, and would involve the elements from all of the services, or could involve elements from all of the services."

Got it? I suppose skeptics shouldn't complain: nothing is in concrete. But then why are the units being organized before "the planning for various contingency uses" begins?

Already, the budget machine is in gear. Some $6 billion is to be spent for new transport aircraft to carry troops and some of their supplies to "distant places." Another $3 billion will buy for the Marine Corps 16 ships to be loaded and pre-positioned in likely areas of conflict. Let us set aside the matter of cost overruns. Think of those 16 ships, floating in convenient proximity to any number of ambitious worthies who might wish to bomb or steal them, their holds filled with tanks in the quiet repose of Cosmoline. Ready deployment?

And of course for the aircraft, if not also for the ships, you need land: not "bases" -- a dirty word now -- but a "presence," one arranged without "formal alliances," says Brown. One wonders if the State Department has a view.

It is easy enough to see why politicians want to seize the moment of the public's attention to appear to be moving decisively toward necessary goals. But it is the responsibility of the president's lieutenants to see that new enthusiasms are sensibly channeled.

President Carter, having yielded his earlier perception that the United States could expect a relatively easy and promising future in the Third-World, now finds there severe economic pressures, an intensifying political instability and a turbulence increasingly leading itself to Soviet-sponsored military exploitation. He spelled out that conceptual basis last week for the military part of his response.

But the first American need surely is to be smart about using the military power it already has. Why, for instance, were those F15s sent to Saudi Arabia a crisis or two back unarmed? How, this week, could officials report that the military action we are considering is "nonviolent"? Why spend fresh billions on new forces when the handling of the old ones is in question?

Are not the likelier chores of an RDF at the lighter end of the spectrum, doable by existing forces properly employed? Brown, to make one point about the RDF, told an anecdote about the dispatch of just one Texas Ranger because "you ain't got but one riot." 'Twixt quip and ship, however, something slipped, because he is now plumping for an RDF that seems only to begin with the outfitting of three Marine brigades.

Some skeptics, haunted by Vietnam, fear an RDF will give us unwise intervention options we won't be able to resist. My main worry is that RDF will give us intervention capabilities unsuited to our real needs. We'll spend, billions, plan, plan, plan, and end up muscle-bound, ill-prepared to bring the right kinds of force into play in a deft and timely way. Brown should go back to that lone ranger.