On Nov. 30, the day the new round of nuclear disarmament talks began in Geneva between the Soviet Union and the United States, Eugene V. Rostow, the director of the U.S. Arms Control and Disarmament Agency, made a speech before the English-Speaking Union in London. It is a speech worth noting.

The sentiments were nothing new from Rostow. But the skepticism this senior Reagan administration official expressed about the effort at capping the nuclear arms race is an important warning sign of the barriers to be overcome--not just in Russia but here-- before the hopes of Geneva can be realized.

The essence of Rostow's argument can be summarized in a series of excerpted quotations:

"The wall between conventional and nuclear war can never be impermeable, no matter how high we make it. Small wars can become big ones at least as readily as in the days when archdukes were assassinated at Sarajevo and Danzig was the center of world concern. It is now apparent that arms control agreements are hardly worth having if they make the world safe for conventional warfare, terrorism and the movement of armed bands across international borders."

Again: While "arms control agreements could result in a somewhat more stable environment, at least in restraining the potential escalation of conventional force conflicts . . . under contemporary circumstances, that is an insufficient goal, and probably an illusory one. . . . The fruits of SALT I and SALT II have turned to ashes in our mouths. The decade which began 10 years ago with high hopes of d,etente became the worst decade of the entire Cold War."

Rostow's argument is that the great danger is not nuclear war but the relentless aggressiveness of the Soviet Union. "There is no blinking the fact that the Soviet Union risks war in its campaigns of expansionism all over the world," he says. And since every war, in his view, is potentially a nuclear war, there can be no real security unless Russia renounces its expansionist goals.

Since earlier arms control agreements have not halted that Soviet imperialism and since the existence of the agreements may have lulled the West into neglecting its own defense needs, the nuclear weapons treaties "have turned to ashes in our mouths."

Believing that, Rostow makes only the most grudging concession to the president's decision to enter a new round of arms negotiations. Indeed, he exclaims at one point, "arms negotiations have no magic in themselves."

Rostow is exceptional in having the temerity to express these doubts at the very moment when the president has launched an ambitious nuclear disarmament plan. But his view is far from unique. There are many like- minded skeptics in the Reagan administration and in Congress who argue that no arms control agreement is worthwhile unless it somehow compels the Soviets also to renounce their habit of creating and exploiting political and military problems all around the globe.

For the moment, Ronald Reagan has embraced the opposite view-- that nuclear war is an evil in itself. He said plainly in his National Press Club speech that limitations on deploying, developing and testing nuclear weapons are goals worth seeking even in-- and perhaps especially because of--the shaky international environment.

He embraced the view that avoiding the use of nuclear weapons in warfare for 36 years is perhaps the most significant achievement of the past generation, one not easily to be dismissed.

It is true that in those 36 years there has been a multiplicity of small wars. There have been countless shifts in the world balance of power, affecting the interests of the Soviet Union and the United States. But all of those conflicts and shifts have not been one-sided. The Soviets--no less than the United States--must reckon their losses along with their gains.

Those calculations are all dwarfed by the overriding fact that, for 36 years, we have avoided nuclear war. We have avoided it because presidents of both parties understood--contrary to Rostow--that arms-control agreements are worth having, even if they still leave us to contend with the risks of conventional warfare, terrorism and cross-border conflicts.

Rostow may believe that the fruits of those treaties are just "ashes in our mouths." But there are many--including, I think, this president of the United States--who rejoice that the globe has not been reduced to nuclear ash, as it might have been without the continuing quest for nuclear arms control.