IN 1976, JIMMY CARTER, newly-elected president, pledged to reduce the size of the nuclear arsenal and cut the defense budget. Three years later, he approved the MX missile, which would add 2,000 nuclear warheads to the American arsenal and boost defense spending by at least $40 billion. How this came to be -- and the almost equally quirky steps the Reagan administration has taken with the MX--is the subject of this book by Australian journalist John Edwards.

The MX, of course, is the new ICBM that the Pentagon wants to start testing next year. The main mission of the MX is to destroy Soviet ICBMs in their protective silos. With 10 warheads per missile, each having twice the explosive power and accurancy of the most modern U.S. missiles today, the MX--so they say -- can do the job. (Whether MX, or any ICBM, can really be that accurate is another question -- one, incidentally, that this book doesn't deal with.)

Another mission of the MX is to prevent the Soviets from doing the same thing to our missiles. To that end, possibly millions of man-hours have been spent figuring out how to base the MX so that even the most uncannily accurate Soviet warheads will not be able to knock it out. Dozens of ideas have been considered -- from tunnels to multiple shelters, to dirigibles, to today's "Dense-Pack" proposal--but nobody has yet found a solution that is both effective and politically practical.

Considering, as well, the dubious value of building a missile designed to destroy Soviet missiles -- a move that may prompt one side or another to launch a preemptive first-strike in a crisis -- Edwards recommends that we simply scrap the MX altogether.

This is a salutary proposal with which I agree. Unfortunately, in telling the story that leads up to this conclusion, Edwards becomes so engulfed in the contradictions of his subject matter that the untutored reader is bound to be more confused than enlightened in the end.

The major problem is that Edwards himself seems confused on the true significance of the MX. At times he ominously intones that the MX is something utterly new, the first U.S. missile designed to dig Soviet missiles out of their silos, the product of a "new" strategy, called counterforce, that has been adopted only in the past decade.

Yet elsewhere he notes that the United States adopted counterforce in the early 1960s and that, while the declared strategy shifted to "mutual assured destruction" a few years later, the officers at the Strategic Air Command--who actually control the weapons--have maintained counterforce throughout.

Edwards is correct in this latter set of statements. In fact, contrary to his other claims, the entire line of Minuteman ICBMs, especially from the Minuteman II of 1964, has been explicitly designed and targeted to destroy Soviet ICBMs. It's just that over the years, the Soviets have built more missiles and have protected them in increasingly more blast-resistant shelters. Thus the United States has built multiple-warhead missiles (MIRVs) and tried to make them more accurate. The MX, in this sense, is yet another instrument to keep counterforce up-to-date.

In short, on the most basic of questions -- how does the MX fit into the arms race, what does it really imply? -- Edwards is terribly muddled. One gets the impression that he started out with a book proposal hyping this new weapon and its new strategy, discovered in the course of his research that counterforce was hardly new after all, added a chapter to that effect but made no alterations in his conception of the weapon -- or of his book -- otherwise.

Moreover, he never really answers the most puzzling question of all: why did Carter and his defense secretary, Harold Brown -- both critical of counterforce upon entering office -- finally approve not only MX but also such plans for limited nuclear war as the infamous Presidential Directive No. 59? One gets a few gestures toward an explanation -- technological determinism, the influence of Zbigniew Brzezinski, pressure from Congress while Carter was trying to sell the SALT II treaty -- but some of these are mutually exclusive, and none of them is conclusively dealt with.

Similarly, he notes that Congress was implacably hostile toward funding more accurate warheads through the mid-1970s and then, all of a sudden, they were accepted by all without argument. Why? What happened? Edwards never tells us.

Not all is lost. Edwards does a competent job of chronicling the bewildering twists and turns of the MX saga -- the endlessly changing plans for how and where to base the missiles, the alternating coalitions and conflicts among leaders in the weapons community, the Air Force, the White House and Congress. His analysis of why the liberals in the Carter Administration voiced no objections to MX is cogent and well-told. So is his tale of the amusing turnaround by two of the Senate's most rabid hawks upon learning that the MX was unpopular among residents of their home states, where the missile would be based, and how they decisively influenced Ronald Reagan to reverse years of Pentagon-White House planning. Most interesting, and original, is his fine detail on the attempts by one presidential commission after another to find the ideal MX basing plan -- each going over the same issues, the same questions, ending with the same sense of futility and hopelessness.

Otherwise, however, Superweapon is unsatisfying. The narrative carelessly flits forward and back in time, lacking coherence, much less drama or punch. The character sketches lack flavor. Too many strategic concepts are introduced with inadequate explanation for the layman and with clearly insufficient comprehension for more knowledgeable readers. Edwards gets many things jumbled, he doesn't seem to understand the limited-nuclear-war theories that he criticizes, and he makes numerous factual errors. A few mistakes are always excusable, but not one every 41/2 pages.

The MX story, while not so momentous as Edwards claims, was -- is -- indeed important. It's good to have a single chronicle of the tale, a shame that it couldn't have been a better one.