If the transaction concerned the purchase of a washing machine the technique would be called "bait-and-switch," and it would be illegal. But as it concerns our security and the cohesion of the Western alliance, it's called "diplomacy" -- and, however objectionable, it isn't illegal.

The issue, of course, is a last-minute Soviet demand that the West Germans scrap their 72 older Pershing I missiles (the "switch") as a condition for a treaty eliminating intermediate missiles from the arsenals of the United States and the Soviet Union (the "bait").

The essence of the bait-and-switch tactic is that the customer is brought to the brink of a purchase only to be told that the deal he was about to make is no longer available -- "the item is out of stock." At this point a higher-priced alternative is brought out in the expectation that the hapless customer, his heart set on a new washing machine, will pay the premium rather than go home empty-handed.

Mikhail Gorbachev evidently believes that Ronald Reagan has his heart set on a treaty eliminating intermediate missiles -- the president's own proposal of 1981 -- and will therefore pay the added price rather than jeopardize the deal. The premium the Soviets are demanding in this case would require the United States to bargain away a weapon that belongs to our German allies. Soviet strategists figure that German confidence in us would be gravely damaged as a result and the already fragmented coalition in Bonn would come under new and dangerous strains. (The Russians caught an enticing glimpse of German angst over these matters when a minor issue, the treatment of shorter-range missiles the United States doesn't have, paralyzed the Kohl administration for weeks while Gorbachev rang propaganda bells throughout Europe.)

The justification the Soviets offer for this mischievous maneuver is the fact that the warheads for the German missiles are under American control (would they prefer them in German hands?), even though the missiles are not. But the treaty that is now taking shape in Geneva does not limit warheads: it limits missiles and launchers. One reason for this is the impossibility of verifying warheads, small and concealable as they are. Another is that the German Pershings reflect what in the special parlance of arms control is known as an "existing pattern of cooperation." As such they have never been included in arms control agreements -- or even in Soviet proposals put forth in previous negotiations, or previously in this one. Gorbachev knows that this new and disruptive demand is unconvincing. That is almost certainly why he waited until he thought unstoppable momentum toward an agreement had the president in thrall before raising the issue of the German Pershings.

The Soviet maneuver is bound to fail, not least of all because it ignores the remarkable, continuing steadiness that the president has demonstrated throughout the course of the negotiations on intermediate nuclear forces (INF). Ronald Reagan knows (and Casper Weinberger and George Shultz are there to remind him) that a treaty requiring us to abandon a longstanding principle and negotiate away the rights of our allies would carry a prohibitive price. That is why he allowed the Soviets to walk out of the Geneva talks in 1983 rather than submit to Soviet demands that British and French nuclear forces be included, along with ours, in a bilateral agreement between us.

Florence Nightingale is said to have commented that "whatever else they do, hospitals must not spread disease." And whatever else arms control agreements may accomplish, they must not undermine the political cohesion of the Western alliance. That cohesion is invariably put to the test when the superpowers negotiate arrangements that affect the security of their allies. Here the Soviets enjoy one of many advantages in negotiation with the United States. Their docile allies -- tranquilized when necessary by a dose of Soviet power -- are not given to effective complaint, even when Moscow treats their security with brutal indifference. Czechs and East Germans may have chafed when the Soviets wheeled in SS-22 missiles as a "response" to American Pershing II deployments in West Germany, but they chafed silently.

For the United States it is essential that arms control negotiations with the Soviets be conducted in a manner that protects mutual confidence between us and our allies. Soviet efforts to lure the president into compromising Allied forces are calculated to destroy that confidence and turn a militarily balanced and equitable agreement into one that is politically damaging to us both.

This is not the first time that the Soviets have proposed a measure that tested American resolve and Allied nerves. At one time or another the Soviets have held an INF zero option agreement hostage to 1) inclusion of British and French forces and American maritime weapons; 2) our SDI program; and 3) Soviet missiles in Asia. Each time we have stood our ground and, in due course, the Soviets have shifted theirs. That is why, after nearly six years, we are close to an agreement that is fair to both sides.

The writer was assistant secretary of defense for international security policy.