In retrospect, it was not the end but the beginning of Cyrus Vance's term as secretary of state that best explains his undoing. He had set out to test the notion, one more accurately called conservative than liberal, that careful step-by-step negotiations of differences with the Soviet Union was the first requirement of American foreign policy. Yet Jimmy Carter immediately launched an agressive human-rights campaign, although anyone who knows anything at all about the Kremlin knows that human rights is an assault on its fundamental legitimacy, and Vance deposited in Moscow an outsized, imbalancing, misguided "deep cut" arms-control proposal.

As it happened, Vance opposed both of these bombshell initiatives, but in vain. An innocent, headstrong and still unchastened president pushed on self-confidently. Thus was the new secretary hemmed in. For the Soviets took Carter's human-rights drive for the destabilization campaign it was -- though Carter to this day no doubt believes he had no such thing in mind. They threw his arms-control package back in his face. Whatever prospect existed of getting up momentum for a term of negotiation shriveled on the spot.

The unhappy result was that events took over: strategic competition in the global areana and bureaucratic warfare at home. Each of these worked against the success of the negotiations approach. Soviet strategic activity provided ammuniton to a contrary approach emphasizing the conduct of a test of strength and will with the Soviet Union. The pulling and hauling with Zbigniew Brzezinski, Carter's national security adviser, sapped Vance's focus, energy and political clout. Many times bloodied, Vance finally bowed.

Carter has the main responsibility. But I find it unforgivable that Vance failed to make clear at the outset that he was secretary of state, that he would not be cowed by an impatient chief, that it was crucial to get off on the right foot during the pause with which the Soviets typically greet any new administration, and that the way to do that was to pick up where the Ford administration had left off and move ahead by stages.

If not to supply just such balance and professionalism, why else was Vance secretary of state? An old Washington hand, he occupied the most prestigious position in the Cabinet and had not yet been marked up by bureaucratic scars. The Soviets had not yet done anything nasty on his administration's watch. He was supposed to be the savvy, cool negotiating hand, the new boy's classy lawyer. He let his client down.

Do a little supposing: Suppose Carter had quickly accepted, without the deep-cut detour, the Ford-drafted Vladivostok agreement that eventually became SALT II. (I have to say here that I favored the deep-cut proposal at the time.) Suppose this had been locked into place while Moscow was still weighing whether to continue the Cuban prowl in Africa. Suppose Carter had not undertaken the human rights offensive that preceded and, I believe, permitted the Soviet Decision to crack down harder on dissidents and emigration. But you get the idea.

No, Vance's problem was not simply that Brzezinski, a hawk and a roughneck to boot, wore down this sensitive retiring soul, the well-born millionaire Wall Street lawyer more accustomed to deference than to mixing it in the streets -- though there is a trace of truth in that.

Nor was the problem simply that Vance's stewardship happened to coincide with the Soviet Union's approach to strategic superiority, a condition the Kremlin was bound to try to turn to its advantage, with inevitable frustrations for Vance -- though there is a trace of truth in that, too.

Nor was it that Vance, in his apolitical establishment way, chose to play to one constituent, Carter, who turned out to have other ideas, rather than trying to cultivate a broad public constituency of his own in the Kissinger manner. The uncommunicative Vance was a disaster at reaching out to address the rising anxieties that were clawing his chosen policy down. But that alone might not have saved him.

The truth is, I think, that in these demanding times the detente idea is worth nothing if it is not advanced in the most skillful and hardheaded way. Any patriot can be hard-liner: all you have to do is stand up and go with the flow. It takes special talent to be a liberal. It is interesting and necessary to ask whether Vance's ideas on dealing with the Soviet Union have substance -- he was a bit too accommodating for me. But if you are Vance, it is much more important to exploit the opportunities given you to make your ideas work. That is where this worthy, earnest man fell short.