Second, the threat of a limited ballistic missile attack against the United States has developed more slowly than anticipated. The 1998 Commission to Assess the Ballistic Missile Threat to the United States warned that, within five years of a decision to do so, North Korea and Iran could acquire an ICBM capable of striking the United States. Seventeen years later, Iran has tested a missile with a range of 2,000 kilometers, and North Korea has tested a missile with a range of 1,300 kilometers—both well short of what would be needed to reach America.

Third, defending the United States against a major Russian or Chinese ballistic missile attack is currently not feasible. A reliable and affordable defense that could protect America against a Russian ICBM and SLBM force that could launch some 1,500 ballistic missile warheads simply does not exist. While the Chinese force is much smaller, numbering several dozen ICBMs, it probably includes countermeasures that would seriously complicate disruption by missile defense systems.

Fourth, for the foreseeable future, offense wins the offense-defense relationship. Offensive ballistic missile technology is far more mature than that of missile defense, and cost considerations favor the offense. Adding fourteen more GMD interceptors by 2017 will require the Pentagon to spend about $1 billion. The Russians and Chinese can each add fourteen more warheads to their strategic offensive forces at considerably less cost. One reason that the Russians are building a replacement for their heavy SS-18 ICBM is to have a missile that can carry ten-fifteen warheads as a means of overwhelming a future American missile defense.

It is important to remember that the other side may not sit passively as the U.S. military develops missile defenses. Other nuclear powers may choose to build up their strategic offensive forces in response, increasing the number of nuclear weapons targeted at the United States (China, in particular, comes to mind). Indeed, it was concern that the ABM systems of the 1960s would spark an uncontrollable strategic offensive arms race that led to negotiation of the 1972 ABM Treaty.

None of this is to say that a future technological breakthrough might not produce a change in the offense-defense equation. Some new technology could be developed that would make defense against ballistic missiles far more lethal, cost-effective and attractive, tilting the equation to favor defense instead of offense. But that breakthrough does not appear to be on the horizon, at least not for the next fifteen-twenty years. And a key lesson of the past thirty-two years is that technology in the missile defense area often does not deliver on its potential—at least not as rapidly, or as inexpensively, as originally thought.

Steven Pifer directs the Brookings Arms Control and Non-Proliferation Initiative.