The downing of two Libyan warplanes by U.S. F14 "Tomcats" last week should come as no surprise to close observers of the Reagan administration's foreign policy. Such confrontations in "third areas" where the United States has clear superiority will probably become the rule rather than the exception. The Reagan administration's emerging policy toward the Soviet Union in a time of growing economic difficulties makes this almost inescapable.

The actual timing of the Libyan incident may have been fortuitous, but it was also revealing. The military success came in the week when the administration first directly confronted the impossibility of simultaneously pursuing its two key goals: economic recovery at home and the rebuilding of U.S. deterrent credibility abroad.

The most compelling evidence of this problem has been the delay in deciding on a new manned bomber and a basing plan for the MX missile in the face of growing pressure from the Office of Management and Budget for cuts in projected defense spending.

The reasons for these delays are to be found not simply in debates over the merits of new weapons systems, but in three grave economic realizations:

Second-quarter statistics just released reveal that the economic situation is of greater long-term gravity than administration economists have believed. GNP declined at an adjusted annual rate of 2.4 percent. Corporate after-tax profits were off 11.3 per cent. Inflation and interest rates are not declining fast enough to spur recovery.

The present U.S. industrial base could not absorb all the military spending contemplated in the time frame scheduled. In the years since the Vietnam buildup, for example, thousands of subcontractors and hundreds of forging foundries have ceased to be available for defense work. Attempting to overcome these weaknesses with rash program incentives will only further overheat the economy--and risk more poor-quality products and cost overruns.

The ultimate economic impact of the Reaganomic budget and tax package will be delayed as a result of congressional compromise. It will also be subject to further damaging distortion, because military spending is highly inflationary. Bullets, bombers and battleships do not "have babies" in the way that investments in machine tools and information technology do. Thus military spending exacts an enormous toll in lost opportunities for rebuilding and modernizing our aging industrial and informational base.

These three economic reservations now being recognized by more and more elements in the Reagan administration make events such as the Libyan encounter virtually inevitable.

Why? According to the Reagan administration's view of foreign policy, our deterrent credibility must be rebuilt quickly. It sees only three possible ways of achieving this, one of which was discredited by experience in the Carter years and another of which is now seen as subject to domestic economic limitations.

The Carter approach consisted essentially of reliance on a moderate military buildup plus frequent exhortation to the Soviets on the virtues of restraint and the vices of aggression and transgression. This policy of "declaratory deterrence," Reagan officials believe, was shown to be bankrupt.

The early Reagan policy of massive arms buildup for what could be called "procurement deterrence" was in large measure a reaction against this. Now the Reagan administration is shifting from procurement toward what might be called "deployment and demonstration deterrence"--a policy that was, ironically, developed by Henry Kissinger, the bete noire of so many Reaganites.

Kissinger, during both the Nixon and Ford administrations, saw that after Vietnam, the country would not, and perhaps could not, sustain high levels of modernizing military procurement. So he sought safe occasions to use American military might as a demonstration of our resolve or deterrent will.

These demonstrations had as their target Soviet proxies or "third areas," but they were aimed chiefly at impressing Moscow. The bombing of Cambodia was one instance. The excessive military response-- however ineffectual--to the seizure of the Mayaguez by what passed for the Cambodian navy was another. All these instances of "demonstration deterrence" were efforts to achieve quick fixes--to get credibility on the cheap.

There is in all this grave danger for American interests. Too slow an abandonment of procurement deterrence could cause irreparable economic harm at home and, thereupon, abroad among our friends. Too rapid and extensive adoption of "demonstration deterrence"--partly as a cover for the abandonment of massive procurement--could result in brush-fire conflicts in tinderboxes such as the Middle East and northern or southern Africa. This would be a recipe not only for continued deterioration of superpower relations but also perhaps for another Vietnam-like war.

What this administration--and NATO and the Soviet Union as well--need most is a new formula for deterrence built not only on the current realities but also on the emerging opportunities of world afairs.

The policies and programs of the past, even in "Tomcat" clothing, even in Rapid Deployment Force formation, will not long suffice, and if pursued, will condemn the Reagan foreign policy to the fate of its predecessors.