In scaling back the military budget, the natural inclination of our leaders will be to preserve the existing mix of forces. That would be a mistake.

There is considerable evidence that our military requirements are shifting markedly in response to Mikhail Gorbachev's new policies and to changing conditions in the non-communist world. By adjusting our forces to the new requirements, we could cut the budget without sacrificing our real security needs.Why not at least ask ourselves whether to devote as much money as we have in the past to nuclear deterrence, to the defense of Europe and South Korea (where we have forces deployed already) and to the projection of forces elsewhere around the world, if needed?

Start with nuclear deterrence. The Reagan-Gorbachev agreement in principle tocut each side's arsenal of intercontinental nuclear warheads in half sent a signal that neither side is thinking about initiating and winning a nuclear war.

If the future is one of declining nuclear arsenals, then why invest in expensive additions to our nuclear forces? The only possible reason to buy new nuclear systems today would be if our ability to deter nuclear war was in imminent danger of eroding. No reasonable person can believe that.

Part of the budget problem could be solved, then, by freezing all new nuclearprograms, including the two mobile ICBMs, the B-2 bomber and D-5 missiles. We could also scale back plans to upgrade weapons.

We should, though, continue with research on the Strategic Defense Initiative. If we ever feel compelled to make our ICBMs less vulnerable, the first fruits of SDI could be an ICBM defense that would cost less than new mobile systems.

The second basic mission, defense of Europe and South Korea, has dominated military planning since the end of World War II. But those days are coming to a close, for two reasons: Moscow wants to save money by substantially reducing its conventional forces in the European theater and Americans are convinced that we are paying more than our share for the defense of Europe.

For instance, our best tank weighs so much that our largest transport aircraft can carry only two at a time. The Europeans might better provide such material.

Thus, a part of the Secretary's budget problems could be solved by freezing procurement of the following: Abrams tanks, Bradley fighting vehicles and heavy artillery that cannot be transported by air; high performance aircraft that require a complex support base; and large aircraft carriers, which are too expensive to risk in lesser wars, like the Persian Gulf.

Worldwide projection of power is the only mission that has involved our forces in combat in the past 44 years. There is little sign it will diminish in importance. There will be more Iran-Iraq wars, more instances of terrorism that call for rescue missions or bombing raids and more political instabilities in areas where we have a stake.

Our extensive preparations for war in Europe have left us ill-prepared for lesser battles around the globe.

This, then, is not an area where trends permit us to economize. Any new spending should go to amphibious forces, to airlift capability and to small aircraft carriers, which could be risked in support of small-scale operations. If no additional money can be made available, this should, at the very least, be the last area to be cut.

The coming military budget cuts could be a blessing in disguise. But only if Secretary Cheney seizes the opportunity to reshape military forces to match the security needs of today, not those of the 1950s.