IT'S TIME to call "time out" in our increasingly chaotic national debate over defense spending.

The defense budget process is designed to accommodate small changes from year to year. But not big ones. When there is a major change -- something on the scale of a $100 billion shortfall in the five-year plan for weapons purchases, an important alteration in allied strategy or an unexpected breakthrough in technology -- we need to stop and think.

If ever such a breather were needed, now is the time. In the past year or so, several profound changes have been introduced into the defense-spending picture, any one of which would justify such a fundamental review. Unfortunately, no such pause is planned. And unless someone insists upon it, we are on the brink of making some extremely harmful decisions that could lead to a less-stable nuclear balance, costlier military hardware and the unraveling of alliances.

Part of the problem is rooted in poor planning; a larger part in the deficit-induced pressure on the defense budget. But part of it is also founded in the introuction of several major factors for which no planning has been done, including:

The INF Treaty. If ratified, the ban on medium-range missiles should provoke new thinking about our defense posture in at least two respects. First, it will focus attention on the lopsided conventional imbalance in Europe. European leaders are already complaining that we have fundamentally altered a successful NATO strategy without having thought through how to compensate for the changes.

The INF accord will also accelerate negotiations toward a START agreement. If the breakthroughs on verification achieved in the INF Treaty can be replicated in the strategic talks, the administration will probably withdraw its current proposal to ban mobile missiles. In that event, budget pressures will probably force a decision between the mobile Midgetman and the MX -- a choice that requires considerable thought and consultation with allies and with the Congress.

The decline in defense spending. It is simply not sensible to believe we can accommodate over $60 billion in cuts in two years -- with the secretary of defense under a current mandate to cut $33 billion more in the next month -- without rethinking the viability of the original strategy on which these requests were made. As a separate but related problem, in the past five years we have ordered tens -- some say hundreds -- of billions of dollar worth of military hardware, and we are not going to be able to pay for it all.

The spending cuts are leading uniformed military leaders, understandably, to circle the wagons around the programs most important to their own services, decisions which may not match what is needed to support the national strategy.

Before adjourning last month, for example, the Congress (with the quiet blessing of some military leaders) came very near to canceling the Midgetman missile program, the centerpiece around which our current national consensus on strategic policy was established four years ago. This wasn't a manifestation of bloody-mindedness or politics. It was a cri de coeur from legislators who don't know what else to do, faced with the enormity of the fiscal problms before us. Surely this is evidence of the need for a time out.

New leadership at the Pentagon. Perhaps the most important new element in the mix is the appointment of a new secretary of defense, Frank Carlucci. He deserves the courtesy of -- and the national interest would benefit from -- a period of thoughtful reflection about where we are headed and how best to get there in light of these recent changes. Our defense program, our alliance relationships and our fiscal integrity are in bad straits. Congressional leaders and the new secretary of defense are conscious of past mistakes and are ready to try, in good faith, to restore coherence to national-security policy -- but they need to pause long enough to do it.

What sort of consensus on these issues might emerge if a domestic summit between appropriate congressional leaders and key executive branch officials were held? The foundation for such a consensus might lie in agreement on some of the basic assumptions that determine strategic stability.

We need, above all, a consensus on SDI, and on the respective roles of offensive and defensive forces in preventing nuclear conflict. Both sides should begin by acknowledging that for the next five to 10 years, there is simply no alternative to our continued reliance on nuclear weapons and the basic strategy of deterrence.

There is little doubt that SDI has been the single greatest factor in leading the Soviet Union to reengage in a potentially constructive dialogue in Geneva. But, as with the overselling of the concept of detente 15 years ago, we now face the risk that SDI will become the holy grail that will do more to polarize the community of thoughtful strategists than to forge a viable strategy for the next generation.

We simply have not thought through the implications of the SDI program for our fiscal stability, the cohesion of our alliances, and the stability of the military equation in U.S.-Soviet relations. But we do have enough data to reach certain conclusions.

Our new strategic consensus should recognize that we cannot expect to put the nuclear genie completely back in the bottle -- we shouldn't, in other words, adopt the goal of the complete elimination of nuclear weapons. That said, it is sensible to support President Reagan's effort to reverse the trend toward building ever-larger arsenals. The nuclear balance can be made more stable at lower levels if the proper weapons -- vulnerable, MIRVed systems -- are eliminated. As this is done, proponents of SDI should accept that the character and scale of any SDI system ultimately found to be effective can and should be reduced commensurate to the level of threat it will then face.

We should also acknowledge that nuclear weapons have been the linchpin not only of our own security but that of our allies as well and that we cannot almost whimsically envision a non-nuclear NATO -- which is where we are headed -- without first thinking through whether it is feasible to wean West Europeans away from their enormous social programs so that they can spend more money on conventional defense.

We need to take time out from the conflicts between the executive and legislative branches to come together on the elements of a new national-security strategy that can engender the support of Americans. Concurrently, we should seek to engender a parallel effort within NATO. We should assemble a small group of "wise men," comprised of thoughtful leaders from within the alliance, who could devote the next year to a cooperative redefinition of NATO strategy. One can imagine such a group being led by Peter Carrington, a man of proven scope and diplomatic skills, and including a half dozen men of the caliber of Henry Kissinger, Jim Schlesinger, Brent Scowcroft, Joseph Tindemanns, Helmut Schmidt, Joseph Luns and others in Europe.

Fortunately we have today -- in Congress, at the Pentagon and among our European allies -- the talent to forge a new consensus. But unless we all agree to step back from the inexorable pressures of the budget cycle, we are destined to sink ever more deeply into the chaos and incoherence that have characterized the past two years. And with that will come the gradual dismemberment of the NATO alliance and ultimate finlandization of Europe.

Robert McFarlane, a former national security adviser, is a counselor at the Center for Strategic and International Studies.