The story of the MX missile is like that of a potted tree that became too big for the house and was planted outside, only to find nothing but rocky ground.

The seeds for the MX, or Missile Experimental, came from a secret 1966-67 Pentagon study called Strat X. Among other things, it addressed the most invulnerable way to deploy the U.S. nuclear offense.

Back then, nearly two decades ago, the United States was betting it could deploy nuclear weapons that could withstand the fire and blast of a surprise attack and shoot back. At least some missiles would survive, it was thought, if enough concrete was poured around them. U.S. bombers would escape by keeping some of them in the air during times of tension. And American submarines prowling the depths with a load of nuclear-tipped missiles were the really invulnerable retaliatory force.

Even the most distrusting war planners in the Kremlin could be expected to regard U.S. offensive forces as primarily for retaliation, not for the "counterforce" role of knocking out Soviet missiles before they could be fired.

American officials in the 1960s bragged about how the United States was able to build much smaller missiles than the Soviets could. The United States, they said, had built deadly precision watches while the Soviet Union had to settle for clumsy alarm clocks when it came to ocean-spanning missiles.

But this confidence melted into concern as U.S. analysts readdressed the implications of the supposed Soviet clunkers pointed at the United States.

Would these big but relatively inaccurate missiles crack Minuteman silos if they exploded over them, locking the U.S. missiles in the ground? Would the Soviet blockbusters blow up on the ground and "dig out" the Minuteman and Titan missiles? And, for psychological reasons, if nothing else, should not the United States threaten the Soviet Union with missiles as big as those pointed in this direction?

By 1974, these Air Force concerns were formalized into a request to Congress for $6 million to look into the possibility of building a missile much bigger than Minuteman. By 1975 this new missile, the MX, was on paper. Then came the hard question: How could this big missile be based so that it was less vulnerable than Minuteman missiles already in the ground under thousands of tons of concrete?

Soviet missiles were becoming so accurate that they opened up a "window of vulnerability" for missiles such as the Minuteman and the Titan, stationary in the ground. Air Force leaders argued for closing this window with a new missile that Soviet gunners could not be sure of taking out, even in a surprise strike. Also, the argument went, pointing blockbusters at the Soviets would make them less tempted to fire their blockbusters.

By the time President Carter came to office in 1977, a consensus had developed that the MX missiles should be kept mobile so they would be hard to hit. Moving the missiles between shelters -- the race-track scheme -- was one proposal; putting them aloft in planes was another. Carter chose a modification of the race-track plan.

Ronald Reagan ridiculed the race-track idea during the 1980 campaign. But after taking office in 1981, he could not find an acceptable place to base the MX, which had gone from a design on paper to a 96-ton steel missile 70 feet long.

Reagan first recommended putting the MX in old Titan holes, and then proposed packing them together on the Wyoming prairie so that incoming warheads would knock each other out instead. He is now back to a previously rejected idea: installing 100 MX missiles of 10 warheads each in Minuteman silos.