FIRST IT WAS the prisoner swap that wasn't a swap. Then it was the hostage ransom that wasn't ransom. And now we may have the nuclear defense that isn't a defense.

"SDI is a purely defensive, purely peaceful technology." So assured Ronald Reagan two months ago at a campaign rally, in which he compared the Strategic Defense Initiative to the development of radar. But underneath Reagan's soothing words of comfort lie the potentially disastrous realities of his Star Wars proposal.

Most of us are aware that the sticking point at the Iceland summit was whether the U.S. would be allowed to deploy a space-based laser "defense" within the next 10 years. Reagan chided Soviet leader Gorbachev for his fears regarding SDI, asking, "What do the Soviets have to fear? SDI is simply an insurance policy for the United States." However, five recent studies--conducted in the U.S., West Germany and the Soviet Union--suggest otherwise.

These studies have demonstrated that a laser system powerful enough to destroy or disable missiles would also be able to destory aircraft, ships and entire cities. The amount of energy required to burn through the shell of an unprotected missile is about 120 times that required to ignite typical construction materials.

In addition, lasers in space could act with lightning speed, igniting 70-80 fires on the earth every second--and that is the capability of just one station. The Reagan Administration has suggested deploying 160 such stations, leading one of the researchers to conclude that, "all of the major cities of either superpower could be targeted for thermal attack by intense lasers with the potential for creating mass fires in all of these urban areas in a matter of hours."

THIS IS SOBERING stuff. Instead of SDI being the benificent shield Reagan paints it to be, it would simply elevate the arms race to a new level of terror. As late as November Reagan still claimed that SDI is, "strictly a non-nuclear defense." However, Robert English of the Committee for National Security has predicted that if research showed the use of SDI for offensive purposes, efforts would be directed toward that goal.

English went on to say that the offensive capabilities might be feasible long before the defensive ones. Many problems encountered by a defensive system--such as the incredibly complex software, a perfectly synchronized network of lasers and the detection of incoming missiles--would not be a problem for a laser system programmed to attack.

IS REAGAN AWARE of the offensive capabilities of SDI? If so, is he hiding his motives behind the smoke screen of a "peace shield," or is he simply naive about how future leaders might apply the technology? According to English, Reagan and his Administration are well-informed as to the attack capabilites of Star Wars.

Unfortunately, Reagan's masterful public relations blitz after the summit managed to convince Americans by a 3-1 margin that SDI is essential to U.S. interests. But what would people say if they knew of its offensive potential?

That's a difficult question. In the 1950s, the government managed to convince much of society that the H-bomb was essential. Those who opposed it were branded as "un-American." Given this and other similar past experiences, there is no guarantee that public outcry would stop deployment.

And the possibility cannot be ruled out that the prohibitive costs of building a defensive system might force the government to concentrate solely on the offensive aspects of SDI. In any case, if the U.S. were to deploy the Strategic Defense-Offense Initiative, we would have spent hundreds of billions of dollars--yet would have gone right back to where we started from. Back to mutually assured destruction, the MAD doctrine raised to the power of two. We would still be hostages of our own destructive technology.