At 10 a.m. Monday, at Warren Air Force Base in Cheyenne, Wyo., the last MX missile will be "deactivated," and a turbulent chapter of history will come to an end. The MX is hardly remembered now; its phase-out began without fanfare four years ago. But in the late 1970's and the '80s - at the height of the United States-Soviet nuclear arms race, amid tensions over the invasion of Afghanistan and the collapse of détente - the MX was the centerpiece of the American military buildup, the object of longer and fiercer debates than any other weapon in modern times.

"It was a defining symbol of an era," says John Pike, director of GlobalSecurity.org. "For its supporters, it was 'peace through strength.' For its opponents, it was 'the mad momentum of the arms race.' Both sides cared about it so much. Now it's going out, and nobody even notices."

It was the biggest intercontinental ballistic missile the United States ever built - 71 feet long, 7 feet, 8 inches in diameter, 195,000 pounds, tipped with 10 nuclear warheads, each sufficiently powerful and accurate (at least in theory) to blast apart a concrete-hardened Soviet missile silo. (Its predecessor, the Minuteman III, held only three, less potent warheads.)

In the mid-1970's, the Air Force saw a growing threat from Moscow. The Soviets were deploying their own monster missile, the SS-18, which could fling 10 large warheads at separate targets. Many worried that enough SS-18's could destroy all our ICBM's in a first strike. With the MX, we could pose the same threat to their ICBM's.