That the Reagan administration has a curiously myopic approach to the problems of world security was never better demonstrated than in a week that was. Consider:

On a Monday, Secretary of State Haig and Soviet Foreign Minister Gromyko wind up two days of "frank, businesslike and serious" talks across the full range of U.S.-Soviet relations and promised more of the same.

On a Tuesday (in Brussels, naturally), the administration's chief arms controller, Eugene Rostow, promises renewed strategic-arms negotiations, while the Pentagon was dishing out for domestic (and European) consumption a glossy, 99-page catalogue of Soviet military might, unrelieved by so much as a glimpse of offsetting American strength.

On a Wednesday, President Reagan leans on congressional leaders at the White House to support his AWACS deal with Saudi Arabia.

On a Thursday, he pledges at a press conference to defend Saudi Arabia against any threat, external or internal, from "anyone that might shut off (Saudi) oil"--as sweeping and explicit an American commitment as we have heard since the early Vietnam days.

On a Friday, along comes the new look in this country's nuclear deployments--"a comprehensive plan for revitalizing our strategic deterrent."

Take just those happenings and the perception is of an administration heavily engaged worldwide: ready to plunge in wherever American vital interests are threatened; working to promote the Free World's common defense; open to arms control and new and safer rules of East-West engagement--but only from a position of greatly increased military power.

But go back to that Tuesday, a full rich day, when the president was addressing the board of governers of the World Bank and the International Monetary Fund, and the perception is altered. For the assembled finance ministers and central bankers of some 140 nations, Reagan had a declaration not of engagement but of American disengagement from the worst part of the problems (hunger, illiteracy, overpopulation) afflicting the so-called developing countries.

Reagan didn't put it that way, of course. But that was the effect on those who have grappled with the problems over the years of his heavy emphasis on "the magic of the marketplace . . . human initiative . . . individual enterprise." The issue for them is not the value of "private enterprise" in principle, so much as its practical application to the plight of those estimated 800 million citizens of the bank's 100 member-states who live in what former World Bank President Robert McNamara describes as "absolute poverty, which is to say, on the margin of life."

McNamara, who gave up the bank presidency in July, after 13 years in the job, is nothing if not discreet. "Exceedingly simplistic" is the most he would say about the notion that "private enterprise can substantially help the very poor countries--the Upper Voltas, the Malis, the Nigers, the Bangladeshes."

But talking to him in his modest office only a few blocks from his old stamping ground, his distress is evident. While publicly praising the administration for its support of a general capital increase recently approved by Congress and at least another year's replenishment of the bank's soft-lending affiliate, the International Development Association, he obviously resents recent assaults by administration officials on the bank's "socialist" inclinations and "welfare state" approach.

Early in his tenure, McNamara considered and rejected the idea that the bank could safely concentrate on the "modern sector" of "developing" economies. Right now, he notes, 70 percent of the population of the developing countries, and about the same proportion of the poor in those countries, live in the countryside. So he moved the bank heavily into rural development lending.

More recently, he has been pushing lending for energy development in poorer countries, to cut their dependence on imports of high-priced oil--and to remove that demand, with its upward pressure on prices, from the world oil market. His emphasis was not on oil exploration in the developing countries nearly as much as on alternative sources: timber, coal, hydroelectric power.

Bob McNamara's plea for expanded, governmental economic development efforts is no new enthusiasm born of his World Bank experience. Far earlier in his career, as Lyndon Johnson's secretary of defense, he took note in a speech, not-well-enough remembered, of the crucial interconnection between our own national security and world poverty; between the consequent social upheaval and conflict between the superpowers. Nor, he insists, does his plea collide with his earlier presidency of the Ford Motor Co.--hardly a training ground for "socialists."

It's just that when he looks ahead to the year 2000 and beyond, he thinks of U.S. security as much in terms of the incendiary effects of global poverty as he does in terms of the threatened vulnerability of an American nuclear weapons system. The same balanced concern cannot be found in the Reagan administration --if one week's performance is any test.