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.

The end of the Cold War offered both superpowers a chance to rethink the utility of their strategic doctrines and, by extension, their nuclear arsenals. Clearly, there was no need for the vast stockpiles of weapons. And new security threats emerged as new technologies offered new options for defense.

The U.S. wanted to harness those new technologies to escape the shadow of nuclear annihilation. Missile defense, long held out as the sane solution to MAD, showed promise. Mr. Bush embraced the program. His decision to withdraw from the ABM Treaty was no surprise; he had pledged to proceed with a missile defense program as soon as it was feasible. He did just that. “As the events of Sept. 11 made clear, we no longer live in the Cold War world for which the ABM Treaty was designed,” the president explained last week. At the same time, Mr. Bush urged the U.S. Congress to develop missile defenses.

The enthusiasm for missile defense has not succeeded in masking its biggest flaw: there is no guarantee that it will work. The testing program has been marred by failures; recent tests have reportedly been successful, but doubts about the validity of the tests have grown as details have emerged. This record has many defense analysts wondering why the U.S. rushed to withdraw from the ABM Treaty. It could have proceeded with testing and development for some time within the parameters allowed by the treaty. But no: Wasting no time, the Defense Department broke ground last weekend in Alaska on six underground silos for missile interceptors, construction that was prohibited under the treaty.

The reason for the haste is that missile defense is a political issue as much as a security one. The ABM Treaty has been in the sights of conservatives ever since they sensed that a viable defense was possible. While there was a moral dimension to their uneasiness with MAD, the real focus was strategic vulnerability. An effective shield would allow the U.S. to act with relative impunity: free from the fear of retaliation, Washington would be able to act as it pleased throughout the world.

That assumed that the system would work — a very questionable proposition. But while the rest of the world is willing to let the U.S. waste its tax dollars as it chooses — and the ABM Treaty is ultimately a bilateral agreement between the U.S. and Russia — there is every reason to be concerned about the collateral effects of U.S. withdrawal. Experts argue that missile defense is vulnerable to decoys; the plans the U.S. has thus far discussed suggest the system will be limited (that is designed to reassure other nuclear powers, such as China, that the U.S. will not neutralize their arsenals).

But the easiest way for China to be sure that it retains a retaliatory capability is to build more missiles. That will encourage India to do the same, which, in turn, will prod Pakistan to respond in kind. North Korea will be watching those developments, as will other governments debating the utility of nuclear weapons. And more weapons deployed means more weapons to protect, more knowledge and materials to control. In other words, nuclear proliferation is a virtual certainty. Ironically, that means that withdrawing from the ABM Treaty is likely to yield greater insecurity. It is a sad end to a cornerstone of the international arms control regime.

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