Guess who's doing his obstructionist best to confound the Reagan administration's efforts to work out a reasonable agreement with NATO allies on the acutely sensitive issue of American nuclear forces in Europe?

You got it: Jesse Helms, the irrepressible Republican senator from North Carolina.Not content with trying to run the State Department by holding up the appointment process for key assistant secretaries, Helms is using the same technique in an effort to make policy for the administration on arms control and defense policy, as well.

The irresponsibility of it becomes apparent when you consider the essential background: a December 1979 commitment by the United States to its allies in the North Atlantic Treaty Organization to pursue a "two track" approach on the modernization of American nuclear weapons deployed on European territory. The deal was that deployment would go hand-in-hand with an effort to negotiate a cutback on the number of so-called Theatre Nuclear Forces (TNF) by both sides, East and West, in negotiations with the Soviets.

The West Germans, the Dutch, the Belgians -- under powerful political pressure for detente, arms control and against any nuclear weapons on their home territories -- feel even stronger about it today. The issue will be high on the agenda of a NATO ministerial meeting in Rome in early May.

For American defense planners, the outcome is critical. Though TNF negotiations fall outside the framework of the strategic arms limitation talks, the European buildup of nuclear force is central to their larger efforts to redress what they see as a dangerous military imbalance between East and West.

In short, U.S. military strategy is conceded, by even the toughest talkers in the Reagan entourage, to be hostage, to some degree, to the sensitivities of the NATO allies. Europe, it's acknowledged, can only be induced to follow the American policy line if the United States is perceived to be making at least some effort to talk with the Soviets about arms control.

Secretary of State Alexander Haig said pretty much just that after a meeting here the other day with NATO Secretary General Joseph Luns. "It is our full intention to proceed in accordance" with the 1979 agreement, he declared. "We are in the process now of doing our homework on this subject with our partners in Europe," he added. "And it will involve preliminary talks with the Soviet representatives with the view toward having negotiations ultimately that would seek to achieve the objective we're committed to."

Now that's pretty explicit. But it's hard to imagine how the administration can really do its "homework" when the two most important positions in the arm of government most directly concerned, the Arms Control and Disarmament Agency, remain unfilled. As of this writing, the White House has yet to confirm publicly its choices, let alone send their nominations to the Senate -- though it is generally agreed that Yale Law professor Eugene Rostow has been picked to be director of ACDA and retired Lt. Gen. Edward Rowny is to be the chief negotiator.

Reason: Jesse Helms wanted Rowny for the director's job and the negotiator's role as well. For a time, that's the way it was shaping up. Rowny had paid his courtesy calls on the members of the Senate Foreign Relations Committee. He won the support of at least one Democrat and all the Republicans, including the chairman, Charles Percy, and Senate Majority Leader Howard Baker -- more than enough votes to carry the committee.

Whereupon the White House had second thoughts. Some sources say it was a matter of imagery -- of not wanting to look too "hawkish" by having retired generals both as secretary of state and in charge of arms control. In any case, Rostow was recruited. Helms and his conservative followers were thought to have been appeased by an understanding that, as chief negotiator, Rowny would have direct access to the president.

Now not even that point is clear. But the irony of it is that Rowny -- a favorite with arch-conservatives for his dogged resistance to SALT II as the Joint Chiefs' representative on the Carter administration's negotiating team -- is not complaining. He thinks valuable time is being lost in getting ready to come to terms with the Europeans.

"I know and respect Gene Rostow," he told me. "It doesn't give me any particular heartburn to report to him -- we'll both be acting under interagency direction for the president in any case."

But as of a few days ago, Jesse Helms' "hold" on Rostow's appointment "still held," according to an official in a position to know. Precisely what further pound of flesh Helms may be seeking, in policy or appointments, is hard to know.

Still harder to understand is the remarkable hold, of another sort, that one Republican senator, however tightly wired into well-heeled, far-right pressure groups, is able to exert on a duly elected Republican president and a newly elected Republican majority on the Senate Foreign Relations Committee.

Note: A recent column inadvertently -- and incorrectly -- left the impression that Defense Secretary Caspar Weinberger "pressured" President Reagan on the AWACS sale to Saudi Arabia in a meeting while the president was hospitalized earlier this month. I regret the implication.