The "chances are very low" that the United States will go to war with the Soviet Union in the 1980s, but Americans are condemned to pay for an arms race with no end in sight, outgoing Defense Secretary Harold Brown said in an interview last week.

This 14th secretary of defense, who has spent his adult life developing and then trying to control nuclear weapons, leaves the Cabinet proudest of keeping the country out of war and most disappointed about failing to get the strategic arms limitation treaty (SALT II) approved by the Senate.

In an interview in his third-floor Pentagon office looking out over grounds that were a battlefield for Vietnam war demonstrators during his first tour in the building more than a decade ago, Brown made these points:

The Soviet Union will keep producing arms well beyond its needs because it has found that the quickest and easiest way to gain influence in the world. The United States has no choice but to match this buildup, meaning a faster arms race unless some new SALT agreement is approved.

Carter administration officials helped derail SALT II by the way they handled the discovery of a Soviet brigade in Cuba in the late summer of 1979. h

The world will continue to be a dangerous place, especially now that there is a scramble for oil and other resources, but this does not mean Ronald Regan will find it harder than President Carter did to keep American troops out of war in his first term.

The U.S. Achilles heel is its economic, not military, condition.

Horror stories about the state of the American military did not hurt Carter much politically nor were the security leaks more frequent than usual for an election year.

As defense secretary, he failed to recognize the need for higher military soon enough and push it through the administration.

There is no way to be sure in advance that the Pentagon has improved its review process to the point that another plan to rescue American hostages from Iran would be foolproof.

Keeping modern enemy weapons from sinking U.S. ships in a war is one of the big technical problems facing his successors at the Pentagon.

As Brown spoke, he sat coatless in armchair to the side of his big desk, which had been used by the World War I commander, Gen. John (Black Jack) Pershing. He struggled at times for a scientific way to answer a question, and occasionally resorted to the sarcasm that infuriated congressional critics during his two tours at the Pentagon.

The 53-year-old Brown, a former Whiz Kid who received his bachelor's degree at 18 and his doctorate in physics at 22, has always made it plain he does not suffer folls gladly. And while this has cost him something in back-slapping Washington, his intellectual adversaries usually acknowledge Brown knows more about defense issues than anyone in government.

"I'm exhausted," complained former White House budget executive Randy Jayne one evening after a long session with Brown. "I've been arguing all day with the smartest man in the world." That was plainly meant to be an overstatement, but did give an idea of Brown's standing in the national defense community.

The reason that "the chances are very low that there will be a strategic war" -- even without the SALT II treaty -- is that "the expected damage to the Soviet Union during the next four years, or the next eight years, is such as to deter any rational person in the Soviet Union from starting a war," Brown reasoned in The Washington Post interview.

But there is no sign, he added, that the Soviets are backing away from the theory that military power equates to political power in the world, whether or not the Soviets fire all those weapons they are producing.

"How else are they going to expand their influence?" he said. "By virtue of their grain exports? By virtue of the quality of their textiles? By virtue of the clarity of their political thinking? None of the above.

"They're doing what they're good at," Brown contended -- producing arms and strong-arming weaker neighbors. "They're doing what they're good at because it has increased their influence. You know when they ship out tanks to Third World nations to use against their neighbors that increases their political influence. And when they invade buffer states, that increases their influence.

"At some point, economic constraints come in, but they're a ways from that. I would say more of the same for the next four years."

At the same time the United States presses "to preserve SALT II limitations" on how many long-range missiles and bombers the two superpowers can deploy against each other. Brown said, "it should continue to build up its military strength." Carter has promised to increase the defense budget by between 4 and 5 percent a year, after allowing for inflation, meaning Reagan will inherit a new Pentagon budget recommendation of about $200 billion for the fiscal year starting Oct. 1, 1981. Such a high defense budget will collide with Reagan's efforts to cut the cost of government.

In terming the Carter administration's failure to get SALT II ratified by the Senate his "biggest long-term disappointment," Brown said "part of the failure was our own doing; the way we reacted to the Soviet brigade in Cuba."

After first saying the presence of the brigade was unacceptable -- "I will not be satisfied with maintenance of the status quo," Secretary of State Cyrus Vance said on Sept. 5, 1979 -- the administration ended up accepting its presence. This caused an uproar in Congress, including the shelving of the SALT II agreement, then awaiting Senate approval.

"I'm disappointed we were not able to damp that down and push through ratification during the fall of 1979," Brown said. The Soviet invasion of Afghanistan in December 1979 put ratification farther out of reach.

If SALT II was his biggest disappointment, he is proudest of the fact that the United States remained at peace for the four years of Carter's term. Brown added that this is "not something for which any individual nor any administration can take full credit because it depends on circumstances to a degree." He struggled for precision when asked if the peace would be harder to hold over the next four years, declaring:

"The chance of having a war during eight years is more than having it during four years. But if you ask if the a priori probability of war during the next four years is more than the a priori probability in the last four, I would say no."

Citing U.S. dependence on foreign oil and inflation, which keeps undercutting Pentagon budgets, Brown said that "even right now the biggest holes in our capability in our security are not purely military."

Asked if -- after years of studying the top-secret information about threats facing the United States all around the world -- he was optimistic or pessimistic, Brown replied:

"The world is a dangerous place. But it has always been a dangerous place. All we're offered is a reasonable change to make things work. Because I'm temperamentally an optimist, I believe that's good enough."