Cyrus Vance has done us all a great public service.

He has given new life and spine to a somewhat rare and weak convention in our nation: resignation in protest on an issue of principle.

Our tradition over many decades -- at least in foreign affairs -- has been curiously different from British and continental practices.

Our dissenters have usually stayed aboard, hoping to fight from within, or have quietly slipped away, giving no hint of their dissent. In other parliamentary democracies, such people often "make waves" by resigning and then moving to the back benches where they can remain vocal and even effective. t

"Effective." That must still be key world in the calculations of appointed officials like former secretary Vance, a very intelligent, genial, seasoned and careful Wall Street lawyer. How, most such people must wonder, can I be most effective?

Well, Vance seems to have answered that question this week by separating himself from a policy he cannot support. His effectiveness within the government has thereby ended, although his effectiveness on the outside is still alive and well -- if he chooses to use it. By his action, though not yet his words, he has done a thing that scores of his previous senior colleagues in foreign policy could not or would not do.

Consider, for instance, those responsible for the making of Vietnam policy. Former secretary of defense Robert McNamara has been widely known for years as a hawk-turned-dove -- who allowed himself to be shunted by Lyndon Johnson (and thereby silenced and cocooned) into the World Bank presidency. His retrospective wisdom has thus been lost to us all until his files or lips are eventually unsealed.

Had McNamara resigned and gone public in 1966 or 1967, the Vietnam tragedy might well have had an earlier and more humane ending.

Similar examples can be found in the Nixon era, especially in regard to those actions in 1970 that led to the destruction of Cambodian civilization. People of conscience did then exit from the government -- but not at high enough levels or with sufficient "voice." Besides allegations of loss of "effectiveness," Vance's resignation-in-protest will inevitably be met by laments from pro-Vance people that he has left the levers of power to such trigger-happy hardhats as Zbigniew Brzezinski. Time will tell; but it seems more likely that Vance's departure, and its fallout, will instead put a brake on White House adventurism and its pushers. What might now be most useful would be the resignations -- and public statements -- of other moderates in the foreign-policy apparatus to help reinforce the Vance decision. In a closely divided party and in a presidential election year, such moves can only have a cautionary effect on the current administration.

But what can be the grounds for such decisions by other loyal Americans to break with their beleaguered president on the apparently unresolvable issue of Iran?

The grounds are, simply -- as many have put it -- gross presidential inconsistency and incompetence. Both have been hovering over our national life for too long now. Both found their most vivid enactment in the hostage rescue fiasco.

Cyrus Vance -- unlike the rest of us -- knew the facts of that adventure, and voted no beforehand. Cyrus Vance -- unlike the rest of them -- had the courage to submit his resignation on principle even prior to the adventure's failure.

Was he a "poor loser" and a "quitter," as they will say in many of those Establishment clubs? A quitter, yes -- of harebrained policies; but thereby also a winner, a person in highest public office who has reaffirmed conscience as the master of ambition.