Reagan proposal's effect on European arms balance

President Reagan is hardly proposing an even swap as he starts his nuclear horse-trading with the Soviets. In offering not to deploy Pershing II and cruise missiles in Europe if the Soviet Union dismantles its SS-20, SS-5, and SS-4 intermediate range missiles, President Reagan is, in fact, proposing that the United States give up 572 nuclear warheads if the Kremlin renounces 1,100, note analysts here. And, they add, the Soviet missiles are deployed, whereas the US ones are still being built.

Leonid Brezhnev's recent offer to freeze deployment of the mobile SS-20 if NATO abandons plans to field Pershing II and cruise missiles suggests that the Soviet general staff and Politburo may not view the proposal with much enthusiasm, they say. Moscow Radio has already branded the President's plan a ''propaganda ruse'' and Tass, the official Soviet news agency, says it cannot lead to an agreement.

The US is currently planning to deploy 108 Pershing IIs and 464 Tomahawk cruise missiles - all with one warhead apiece - in Britain, West Germany, Italy, the Netherlands, and Belgium to offset the threat posed by 250 SS-20 missiles, 315 SS-4s, and 35 SS-5s. While both the SS-4 and SS-5 are single warhead weapons , the SS-20, which reportedly has a range of 3,000-5,000 miles, carries three ''very accurate and independently targetable warheads,'' according to the Pentagon. All three missiles threaten European population centers.

By contrast, the US has no missiles in Europe that can hit the USSR. Its 108 Pershing 1A and 36 Lance missiles have ranges of 160 miles and 70 miles respectively.

In making his proposal, President Reagan rejected Soviet claims that a balance of intermediate-range nuclear forces already exists in Europe. ''That assertion is wrong,'' he said. ''By any objective measure . . . the Soviet Union has an overwhelming advantage, on the order of 6 to 1.''

Secretary of Defense Caspar W. Weinberger underscored this point at a breakfast with reporters just before the President's speech, pointing out that the SS-20 ''can hit anything in Europe'' even if it is withdrawn beyond the mountain chain.

Secretary Weinberger denied that the President's proposal constituted a ''propaganda ploy'' or that it was designed to damp down antinuclear demonstrations in Europe. ''It should bring a great deal of hope to the world,'' he said. The demarche had been months in planning, the secretary added.

Questioned about verification, Weinberger conceded that it would be the ''vital part'' of any agreement that might be reached. He said he did not know whether spy planes and satellites would be used to ensure compliance, but observed that verification would have to be achieved ''by some means that would give complete confidence to both sides.''

President Reagan also announced that he will seek strategic arms reductions with the Soviet Union. ''Only such progress can fulfill the hopes of our own people and the rest of the world,'' he said, adding that ''to symbolize this fundamental change in direction, we will call these negotiations START - Strategic Arms Reduction Talks.''