The United States officially withdrew from the Antiballistic Missile Treaty last week. Exactly six months to the day after it announced that it would take that fateful step -- the period stipulated in the ABM Treaty -- the administration of President George W. Bush turned its back on the strategic doctrine that guided Washington through the Cold War. The world now enters a new and uncertain phase as the U.S. pursues defensive systems to protect itself from the threat of nuclear weapons.

The ABM Treaty was signed in Moscow by U.S. President Richard Nixon and Soviet leader Leonid Brezhnev on May 26, 1972, and entered into force four months later. The treaty barred the two countries from deploying systems that could defend their entire territories from intercontinental ballistic missiles. It also banned development, testing or deployment of mobile land-based, sea-based, air-based or space-based antiballistic missile systems.

The treaty was the foundation of strategic stability during the Cold War. The absence of defensive systems meant that the two superpowers held each other hostage; any military conflict between the two sides risked escalation to a nuclear exchange that would have resulted in horrific casualties -- tens, if not hundreds, of millions of dead -- on both sides. The logic was MAD: mutually assured destruction. On paper, it was immoral. In reality, it worked. The two superpowers fought a cold war for decades, but never a hot one. Fear of a nuclear exchange was the reason why.