That SALT bird

President Reagan is in a sense correct in saying that the unratified SALT II treaty legitimizes the arms race. It does that by permitting both sides to go on building certain weapons systems. In the best of all worlds it is certainly better to pursue deep reductions of mutual arsenals. But it is still a pity that the positive aspects of the treaty cannot be nailed down permanently even while US and the Soviet Union pursue START talks. SALT and START are not incompatible.

Mr. Reagan stated in his press conference last week that the parts of the SALT II treaty now being informally observed by both sides have to do with the monitoring of each other's weaponry. That is not quite accurate. Monitoring goes on all the time, with or without SALT II. The point is that the parties are not formally discussing compliance or other problems arising under the treaty in the forum where such matters normally are brought up: the US-Soviet Standing Consultative Commission. That commission now deals only with SALT I. If critical problems were to arise--if the US did, say, suspect the Russians of exceeding the SALT II limits--there would be no avenue for rectifying the situation.

In actual fact, there is informal compliance with the treaty in areas which Mr. Reagan chose not to mention. The Russians have not dismantled 250 missile launchers as called for by the pact, for instance, but they have phased out their old submarines as new ones have been deployed. Neither side has exceeded the limits on total launchers.

There may be risks in signalling the Russians that the US does not regard compliance with SALT II as anything but temporary (one high US official recently used the words ''for the time being''). Consider the matter of warheads, for instance. SALT II places limits on the number of warheads which Soviet missiles can carry. The heavy SS-18, which represents a threat to US land-based forces, is allowed 10 warheads. If the Russians conclude the US is no longer interested in holding to SALT II, they could break out of the treaty and, by further launch tests (in violation of the agreement), double or even triple the number of warheads on the SS-18. Think of the threat to the US Minuteman missiles then! According to arms experts, there is nothing comparable the US could do quickly. SALT II, in other words, puts a cap on the capability of the big Soviet missiles and thus mitigates the Russians' advantage in throw weight.

No one thinks SALT II is an ideal treaty. Even the Joint Chiefs of Staff termed it only a modest step at the time it came up for debate in the Senate. But the US should be careful about discarding an agreement that serves a useful purpose. Instead it should consider how the treaty might be revised to take account of US concerns - a move now advocated by former Secretary of State Henry Kissinger, whose initial support of the pact was lukewarm. Perhaps SALT II could be made something less formal than a treaty but more than an unratified treaty, and something which would distinctly have the Reagan stamp on it.

The President clearly has his domestic flanks to protect. It would be politically difficult for him to embrace the treaty as it stands. But he also has the US national security to protect. Even while moving forward on a new strategic arms agreement that actually reverses the arms race--a goal the American people will heartily applaud--he could see to it that the good elements of SALT II are preserved. It bears reminding that it took four years to negotiate SALT I and seven years to hammer out SALT II. With technology advancing and arms control problems intensifying, it could be many years more before START I is achieved.

Why not have that proverbial ''bird'' in hand--refreshed with a few new feathers so it is no longer the ''unfriendly'' creature the President says it is?

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