Ronald Reagan deserves great credit for opening up his heretofore sacrosanct defense budget to cuts--not absolute cuts, mind, but cuts in the rate of extra growth beyond the very substantial growth earlier planned by Jimmy Carter. It is reassuring to see this evidence that he means what he has long said about economic strength's being the foundation of national security.

Nor does it seem that he is compromising his Pentagon ambitions just for a short spell. No less beady-eyed an observer than James Schlesinger suggests that, "Substantially to reduce the out-year deficits, given the growing difficulty in achieving non-defense cuts, would probably require that some three out of four dollars in reductions come from defense." Put that in your pipe and smoke it.

But even as Reagan is drawn to the stern necessity of meshing his economic and defense plans, another difficult and unavoidable task looms up before him. It is to blend in foreign policy, and in particular, policy with respect to the Soviet Union.

To see the problem, you have merely to hearken to the words President Reagan spoke on Sept. 3 during the very period in which he was accepting the need to roll the annual increase in defense spending back toward the Carter 6 percent (in real dollars). If the Soviets continue their military buildup, he declared, "they will be in an arms race which they can't win."

For what it's worth, I have always felt it was loopy to hail an arms race and to regard arms building as a suitable arena for Soviet- American competition. But Reagan has held a different idea. Through a faith in the free enterprise system, or through a belief that circumstances would compel the Soviets to accept inferiority, or something, he has welcomed such competition.

Cheered at the prospect of engagement, confident of victory, he has expected the Kremlin at some point to signal uncle and to ratify, in a Reagan-written arms control agreement and in acceptance of American political designs, its permanent status as No. 2.

In any event, having freshly challenged the Soviets to an arms race "which they can't win," the president now finds himself conceding in the budget numbers, where it counts, that the United States may not be able to conduct that kind of arms race after all.

It is not that the 6 percent growth he may be settling for delivers so much less real defense capability than the 8 percent growth he had hoped for. It is that 8 percent has the aura of a godlike president operating without economic or political restrictions and 6 percent suggests that the president, though still a formidable figure, is bound by certain constraints.

Some people are chilled by Reagan's return to the realm of the mortal. They are already advertising their alarm that the Soviets, the Europeans and the rest will see the new Reagan budget approach as a retreat to Carter-style wishy-washiness. Perhaps. My own tendency, though, is to welcome the new approach as a turn that makes possible certain foreign policy steps that were less likely before.

I don't believe for a minute that the Reagan foreign policy will now be Carterized. But I do think it appropriate for the administration to adopt a less defiant tone in speaking to its adversaries and a humbler voice in addressing its allies. Whether the administration can put itself back on the arms control track is uncertain-- most of its leading figures distrust the SALT process profoundly--but certainly it has no call to keep challenging Moscow to an arms race. Diplomatic strategies based on the Reagan premise that the United States is on the way to building overwhelming positions of local military strength everywhere must yield to more realistic calculations.

This need not be any serious embarrassment to Ronald Reagan. He will still have a massive and growing amount of sheer military strength to dispose of, and he is better fit than anyone else to explain to the public his sage, time-mellowed views on the relationship between economic solvency and national power.

All along, moreover, Reagan has underused his greatest foreign policy asset, his credibility as a hard-liner. He's no magician, but his image is worth, quite literally, billions. For it's not just hardware tallies and dollar totals that matter. To the extent that projecting toughness and constancy is our problem, a Ronald Reagan with a 6 percent defense increase goes an awful lot further than a Jimmy Carter with 8 or 10.