'In foreign affairs it's easier to be a fireman than an architect'

George Shultz says the job of the president's commission on Central America is to ''take a long-term look'' at US policy. It will be a great day when this principle can be put into practice - worldwide. Up till now, US foreign policy has been reflexive and disjointed - reflecting the primal instinct of each administration to pass on the thornier issues to its successors.

Take nation-building as a case in point. Since World War II the territories spun off in the breakup of European empire have coalesced into a multitude of states and statelets, bringing the total membership of the international family close to 150. By international convention, all are ''sovereign,'' but none is truly independent, for independence is the mirror of power, and power is never absolute.

The United States, greatest power on earth, was unable to insulate its citizens from the aggravation of the Arab oil embargo. Vulnerability to outside pressure increases as a state gets smaller and weaker, to the point that large areas of Africa, Latin America, east Asia, and the Middle East are political cauldrons seething with rival forces.

Such areas of instability were always hazardous for their own residents, but in the nuclear age they pose the unacceptable risk of war to the world at large. Political fragmentation is as ubiquitous as Mark Twain's weather, and almost as hard to deal with. Everyone recognizes the powerful advantages of regional unification:

* More efficient economic development (note how extraneous political factors have interdicted Arab pipelines, closed the Suez Canal, and polluted Gulf waters);

* Cheaper government per capita;

* More domestic tranquillity;

* Fairer distribution of wealth among areas unequally endowed with natural resources;

* More sophisticated technology;

* Most important of all, greater protection from foreign intrusion; ''divide and conquer'' and ''in unity there is strength'' are two sides of the same coin.

It follows that the US should pursue a coherent policy designed to promote regional unification around the world.

It should - but it doesn't.

It has supported the valiant European effort to shake off the ancient heritage of civil war. It even heeded the wise advice of the late Charles Yost when, as ambassador to Syria, he urged Washington to give Egyptian-Syrian union a fair chance. But in general the US has turned its back on its revolutionary antecedents and stubbornly espoused every extant national entity, however ramshackle. When one of these states is threatened, particularly by agencies with some Soviet association, Washington's Pavlovian response is to sound the trumpet and ride to the rescue.

The current manifestations of this mentality are joint military exercises to shore up Egypt, Somalia, Oman, and Honduras, military aid to Salvador, Egypt, and Chad, AWACS to defend Chad and Saudi airspace, military advisers in Salvador , Chad, and Honduras, military access agreements with Somalia, Oman, and Kenya, marines in Lebanon, and a not-so Rapid Deployment Force to oppose an enemy yet to be determined.

This acrobatic posture is strong on damage limitation (military) and weak on attacking root causes (political). In foreign affairs it's easier to be a fireman than an architect. Political ferment seems to be a permanent human condition. We can't stop it, we can't even control it, but we might do better at modulating it.

If the roof blows off in Chad or Salvador, the arena is wide open for communist or fascist intrigue, but enlightened US policy must concentrate on rebuilding from the ground up. Over the long term (Shultz's words again), the best insurance against regional instability is political unification.

Any progress in this direction has to overcome the omnipresent vested interests, which are quick to don the sacrosanct mantle of national sovereignty to conceal the opportunities for personal and factional aggrandizement that every national administration affords. Exploitation often thrives in the fetid atmosphere of communalism, which carries the primitive us-them dichotomy to its ultimate absurdity.

The current fragmentation of the Terran policy into a plethora of sovereign shards may be a necessary stage in human evolution, but it is a dangerous one that must be accelerated if possible.

Apparently the Sandinistas want to socialize Central America and Qaddafi wants to Islamicize North Africa. The burning question in Washington, it seems, is ''How do we stop them?'' If we take the long view, we have to ask a different question: ''Where does future stability lie?''

Policymakers don't like this question because it takes more time and effort and political risk to answer. For instance - the disadvantaged people of impoverished Chad need a wealthy partner. If neither France nor the US can spread itself that far, which seems evident, then we should look to Libya or Nigeria or somewhere else.