Everything uncertain and risky in the U.S. intervention in the Persian Gulf is summed up in the word ''open-ended.'' You hear it intoned gravely and applied not simply as a description of the character of the American involvement but also as a telling indictment of it.

Open-ended, of course, means Vietnam. It means going in innocent and unsuspecting, getting drawn in deeper, watching weakly as costs go up and support goes down, finding the original rationale eroding, being unable to project a sense of clear purpose and finally suffering a crashing defeat.

Fear of open-endedness is what helped Ronald Reagan determine to cut his losses quickly once trouble broke out in Lebanon. It has contributed substantially to his reluctance to lance the Nicaragua boil with an American military action. It made his quick in-and-out invasion of Grenada the Reagan-era model of direct intervention. It is the political ground under the Reagan Doctrine of supporting local insurgencies against the Marxist-Leninist regimes established in the 1970s.

Caution born of fear of open-endedness has largely triumphed over whatever degree -- not all that great, I suspect -- of eagerness for military action that Reagan brought to the pursuit of anticommunism. And it has shaped American policy in the Persian Gulf in crucial ways.

There officials find themselves in a bind that Vietnam made familiar. The requirement to calm the public produces assurances that the war -- which has already lasted almost a decade -- is not going to endure ''that long,'' as the State Departmentputs it. But the requirement to be taken seriously by other countries pushes officials to convey that the United States can stay the course.

Iran has picked up on the American ambivalence and warns Gulf Arabs that, since the United States is not in the Gulf for the long haul, they had best come to terms. It is a clever pitch, given the Arabs' chronic anxieties -- freshened by the Reagan pullout from Lebanon -- about American constancy.

Iraq may have picked up the same American ambivalence. It seeks to avoid being seen as practicing entrapment, but certainly it has an overwhelming interest in sharpening the American interest in ensuring that Iran does not win the war and does not move on to new fundamentalist glories.

The Reagan administration did not get into this latest nervous phase in the Gulf deliberately, to take advantage of an opening for strategic gain or peace. Quite the contrary: Reagan started beefing up the American fleet in the Gulf last January (reflagging came later) hastily and erratically to protect American standing with the Arabs from the fallout of his secret dealings with Iran.

Still, the administration has since added some prudence to its effort to contain Iran and maintain the traditional Western presence in a vital region. It has extracted increasing cooperation from antsy Gulf Arabs. It has helped four or five oil-consuming nations to see their own reason to protect Gulf shipping. It has done its part to launch a United Nations diplomatic initiative, which, however tenuously, offers possibilities of cease-fire and settlement that did not exist earlier. It has countenanced -- having no choice -- the mixed blessing of a Soviet peacemaking role.

In the Gulf there are no results yet to point to. But at home a small miracle has taken place under a scandal-weakened lame-duck president facing a Democratic Congress -- no less. Everyone regrets the origins of the new Persian Gulf involvement, nitpicks tactics and worries of open-endedness. But there is an implicit if ragged acceptance that we cannot arbitrarily limit ourselves to projects that are short, sure and sweet, and the basic involvement is accepted and understood.

Maybe that's the antidote to the curse of open-endedness. The critics are right: things could get much worse. What if, for instance, Kuwait asked Washington to protect a lot more than its tankers? President Reagan dulls the force of the criticism, however, by respecting a popular consensus that demands of an involvement that something of value be at stake (oil, alliance, strategic presence), that military actions be sensible and that those who stand to share in the benefits of American exertion play a decent supporting role.