The crucial gap in national security is not in missiles or anything like that. It is the gap between foreign policy and economics. Practitioners and observer alike tend to work on opposite sides of a wall. But a connection must be made. It is fateful, and it is summarized in the question: Can we afford our foreign policy?

President Reagan, unfortunately, is faking. He has rushed past the question of whether we can afford his foreign policy -- his defense budget and the international commitments he would have us sustain. He simply asserts it. The public, in its anxiety about the Soviet threat, has not demanded that he prove it. He is allowed to make defense spending projections like these: 1981 $162 billion 1982 $189 billion 1983 $226 billion 1984 $256 billion 1985 $304 billion 1986 $343 billion

The assumption implicit in these figures, moreover, is that the inflation rate will fall to 5 percent. Isn't that out of this world?

I have just read a couple of eye-opening essays that break through the security-economy wall. One is a forthcoming book, "Solvency: The Price of survival," by James Chace, managing editor of Foreign Affairs magazine, and the other is an article, "Inflation and American Power," in the new Foreign Affairs, by David P. Calleo of the School of Advanced International Studies of Johns Hopkins University.

Chace and Calleo end up in somewhat different places, of which more below. More important, they start in the same place, with the conviction that there is an iron link between our profligacy at home, especially in the matter of our seemingly relentless inflation, and the weakness that both our allies and adversaries perceive abroad.

Chace updates Walter Lippmann's notion of a "solvent foreign policy": "bringing into balance, with a comfortable surplus of power in reserve, a nation's commitments -- economic, political, military -- and a nation's power." Measuring our stagnant productivity, scandalously high oil imports and volunerable currency against Reagan's expansive and expensive foreign policy goals, he concludes that the United States is increasingly "insolvent" and that the urgent need is to define more precisely what are the "vital interests" we would fight to uphold, and to accept the necessity of deciding whether we need, for instance, items like the MX. He warns of the "Vietnam economic syndrome" -- conducting foreign policy on credit.

Calleo argues that to control inflation we need not only to do battle over the domestic component in the budget but also to reorder the priority between domestic and foreign goals, either by sacrificing more at home to pay for an enhanced international position or by scaling down our world commitments. He fears Reagan's "evasive fantasies will lie with 'supply-side' economics and a reassertion of foreign policy activism." His own major recommendation is to leave Europe's ground defense to the Europeans.

One can differ, as Chace and Calleo do, on policy prescriptions. But it seems to me impossible to blink away their focus on the economic underpinnings of national security. This year, carried by his electoral breeze, Reagan can, and probably should, get the military numbers he wants. As he escalates defense spending, however, he casnnot and should not fail to come under hard challenge.

It is one thing to spend, as now, 25 percent of the budget on defense. Reagan is moving toward 38 percent regardless -- here is the key -- of whether he slays inflation. Consider that just in the last quarter of 1980 the Pentagon's cost estimates on 47 major weapons systems rose by $47 billion -- more than the total Reagan seeks to trim from social programs.

Senior officials, Michael R. Gordon of the National Journal reports, are "well aware of the potential pitfalls stemming from their optimistic inflation assumptions. . . . In effect, they're gambling that if the administration's economic plan doesn't work miracles, the public will support substantial supplemental defense appropriations."

Somehow the questions now seem to come mostly from people who can be pigeonholed as Carterites, sore losers. This is wrong. The most rock-ribbed, defense-minded and conservative among us should form up a parade of skeptics, and lead it. Chace and Calleo provide a text.