Who is more vulnerable?

The current debate over American weaponry provides an example of how in American politics campaign rhetoric can plague the winner. During last fall's presidential campaign the Republicans popularized the idea that the United States would shortly arrive at ''a window of vulnerability.'' This means a period of several years during which the US would presumably and possibly be at the mercy of a Soviet nuclear ''first strike.''

The alleged moment has not yet quite arrived. The US still has a lead over the Soviets in the number of nuclear warheads which can be tossed back and forth between the two superpowers. The US at latest count has 9,000 as compared to 7, 000 for the Soviets. That is because, while the US has fewer vehicles which can lift warheads from the US to Soviet territory, more of these US vehicles carry multiple warheads than do the Soviet vehicles.

But the Soviets are catching up in the technique of mounting multiple warheads on their vehicles. They also are supposed to be learning how to make their warheads more accurate. In theory now both can land these unpleasant things within 300 feet of a given target.

The US now has 1,052 holes in the ground in US territory from which intercontinental nuclear vehicles (missiles) can be launched. In theory the Soviets by 1983 or 1984 will have fitted enough high-accuracy systems to enough of their long-range missiles to be able to knock out most of the American fixed, land-based missiles.

That sounds bad to any American ear. And it is certainly not a prospect to be welcomed. It sounds worse when it is added that the Soviets have more of their missiles based on Soviet territory than Americans have on American territory. They have 1,398 to the American 1,052.

But does that mean that the Soviets are actually better off (as such things are reckoned)? Not at all. Actually, they are worse off because their land-based missiles are just as vulnerable to American missiles as the American holes in the ground are vulnerable to Soviet missiles. They present more land-based targets than the Americans do.

And the Soviets do not have anything else of first-rate value in the way of strategic weapons. They have 150 long-range strategic bombers, but these are relatively slow, lumbering things which could reach the US and get back only with refueling. It is doubtful that many, if any, could ever reach a target against serious opposition. They are inferior to America's 316 B-52 intercontinental bombers.

The Soviets also have 989 missiles mounted in 84 submarines. But 54 submarines are short-range. Also the Soviets have yet to overcome the noise and turnaround problems. Soviet submarines are easily detected and tracked as soon as they move out from their own protected coastal waters into the open ocean. US submarines have achieved remarkable ability to keep quiet and avoid detection. Equally important is that, while the Soviets have 84 strategic submarines, they seem never to be able to keep more than 10 on patrol at any one time. American maintenance is so much better that more US strategic submarines are out on patrol at any time although the US has only 36 such ships to the Soviets' 84.

Thus the US has a decided advantage in both airborne and submarine-launched strategic missiles. And these are not involved in the ''window of vulnerability.''

President Reagan has ordered 100 of the new MX-type missiles. These carry 10 warheads each. The present Minuteman III carries only three. The Titan carries one. The first of the MX missiles are to be substituted for the old Titans and the earlier Minutemen. They will be in the ''window of vulnerability.'' But a point frequently overlooked is that, even if all American land-based strategic missiles are vulnerable, they are only about half of the total strategic weaponry of the US. The Soviets have most of their effective eggs in their land-based basket. So, in terms of the latest new missiles capable of taking out other land-based missiles, the Soviets are even more vulnerable than is the US. They too are vulnerable to a ''first strike.''

If the Republicans had not made so much of the ''window'' during the campaign they would not be under quite so much need to spend money now trying to close it. But their own past words push them to spending more than the true military situation probably requires. And this in turn is making their economic problems more difficult.

John F. Kennedy claimed a ''missile gap'' when he was running against Richard Nixon in 1960. Once Mr. Kennedy was in the White House and knew there was no ''missile gap'' he dropped the subject. It disappeared from politics and the budget. Mr. Reagan would be wise to imitate his predecessor.

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