WHY INDEED is the Soviet Union engaging - in Europe, a peaceful region where there is no threat at all to the Kremlin or its allies - in a "continuing buildup of unprecedented proportions"? Why indeed is the Soviet Union deploying against Western Europe the mobile missile known as the SS20, which "is a new departure in destructive power and represents a substantial increase in the nuclear threat of the Soviet Union"? Vice President Mondale bluntly drew the attention of the U.N. special session on disarmament to those Soviet programs on Wednesday, and he was well advised to do so. The responsibility for building up armaments in a fashion that kindles tensions must be placed where it belongs.

The United States, to be sure, has not been at rest in its European military preparations. But while the Soviet buildup centers on tanks and planes for offensive actions and on nuclear missiles meant for intimidation or attack, new American programs involve anti-tank and plane weapons and a prospective nuclear weapon, the so-called neutron bomb, designed to be used only on NATO soil in self-defense. There is no fair comparison, no matter what the Soviet foreign minister may contend otherwise in his speech today.

It may be said that Mr. Mondale, in his remarks on the Soviet Union, was importing into an international forum an American preoccupation better pursued outside it. We find the point unpersuasive. Nothing aggravates more the global overarming that the special session is meant to ease than Soviet-American rivalry. That is what has led Moscow to flood vulnerable states in Africa with arms - and with advisers and mercenaries. The worst part of the turbulence now affecting the continent is the result of this Soviet policy.

If there was an American propaganda purpose in the vice president's speech, there was also an appeal to the world community to use its moral and political influence to hold the Soviet Union, no less than the United States, to restraint in the building, wielding and transferring of arms. The nations composing the U.N. majority cannot hope to gain therelative tranquillity they need to tend to their development priorities - priorities, by the way, that Moscow resolutely ignores - so long as the Kremlin prowls around searching for opportunities to buy itself local advantages with planeloads of tanks and guns.

It is a continuing paradox of Soviet-American relations that, notwithstanding American distaste for various Soviet policies, the United States has an over-arching interest in reducing the risks of nuclear war and slowing the accumulation of strategic arms. That is why the United States must pursue the strategic-arms talks moving, in New York and Washington, toward something of a climax even as Americans react to Moscow's African adventures and to its latest bursts of repression at home. There are steps Washington can take to counter or anticipate Cuban and Soviet moves in Africa. President Carter made a good case yesterday in Chicago, for instance, for removing the congressionally imposed aid curbs that put American policy in Africa in something of a straitjacket. But at the same time Mr. Carter underlined, usefully, that, although Soviet policies may make it harder to conclude and sell a SALT agreement, SALT remains a vital American interest. In the heat of political batttle at the United Nations, that is a good thing to keep in mind.