THE uproar over the expected North Korean missile test brings the question of missile defense, once again, to the fore. Before Ronald Reagan gave his 1983 "Star Wars" speech, both Democrats and Republicans believed that the only way to counter the USSR's nuclear threat was to deter aggression with

After launch, a nuclear missile is most vulnerable in its boost phase, when moving at relatively slow speeds and releasing a huge amount of heat that makes it easy to target. But it can also be intercepted in its mid-course phase, when the warheads, along with any decoys, have been deployed; and in its terminal phase, when the warheads dive toward their targets and the decoys burn up in the atmosphere. Reagan's Strategic Defense Initiative included ideas that would intercept missiles in all three phases.

In spite of limited budgets and the ABM treaty's restrictions on testing and development, the Strategic Defense Initiative Organization (SDIO) made impressive progress. It found that a space-based defense that could hit a missile in its boost phase was technologically possible. This project, known as Global Protection Against Limited Strikes, was ready to move into testing and production in 1993, when it was canceled by Bill Clinton's first defense secretary, Les Aspin, who reportedly boasted that he was going to "take the stars out of Star Wars."

The 1994 midterm election changed everything. GOP leaders, now in the majority, pushed hard for Reagan's vision. Clinton, recognizing the political futility of openly opposing missile defense, decided to let the SDIO (now renamed the Ballistic Missile Defense Organization) proceed with plans to build a basic missile defense while keeping within the limits of the ABM treaty. The primary focus of this project was to be a ground-based interceptor that would hit warheads in mid-course, the phase in which they are hardest to target.

George W. Bush took office in 2001 committed to building a missile-defense system "as soon as technologically possible." His new secretary of defense, Donald Rumsfeld, had chaired two panels on missile defense and military space activities. One of those, the Commission to Assess the Ballistic Missile Threat to the United States, had concluded in 1998 that "the threat to the U.S. posed by [enemy missiles] is broader, more mature and evolving more rapidly than has been reported in estimates and reports by the intelligence community." The commission estimated that North Korea's Taepo Dong 2 missile could reach Alaska and parts of Hawaii. It also wrote that a lightweight version of the same missile could place at risk "western U.S. territory in an arc extending northwest from Phoenix, Arizona, to Madison, Wisconsin." It noted that Iran was moving forward rapidly with both missile and nuclear-weapon technology. Finally, it said that these states had the ability "to deceive the U.S. about the pace, scope and direction of their development and proliferation programs."

After 9/11, it became clear that the U.S. could not take any chances. It is sometimes said that missile defense has nothing to do with terrorism, but that claim loses plausibility as Iran, the world's greatest terror sponsor, pursues both nuclear weapons and advanced missile technology. In December 2001, Bush decided to scrap the ABM treaty and proceed with an unrestricted version of Clinton's missile-defense plans. Despite a number of test failures, the Ballistic Missile Defense System is now in place in rudimentary form. It includes ground-based interceptors in Alaska and California under the Army's 100th Missile Defense Brigade. A number of highly effective SM-3 sea-based interceptors, which can be launched from Aegis cruisers and destroyers, are also part of the system. Meanwhile, we are making good progress on the Terminal High Altitude Area Defense system, which will supplement our Patriot missiles. Also promising is an airborne laser, mounted on a Boeing 747, that will be able to hit Scud-type missiles in their boost phase. But it's unlikely that either of these systems will be ready before 2010.

Internationally, missile defense has been embraced by the Japanese, who are buying a number of advanced Patriot missile units, as well as SM-3 sea-based interceptors for their own Aegis destroyers. Israel has a fairly modest defense against ballistic missiles called the Arrow, developed with, and partly paid for by, the United States. In Europe, Germany and Italy are participating with the U.S. in a project based on the Patriot. Britain and Denmark have both given the U.S. permission to use existing American radars in Yorkshire and Greenland for missile defense. Even the French, who have long been extremely hostile to missile defense, are hoping to give their new Aster anti-aircraft missile a defense capability.

By withdrawing from the ABM treaty, the Bush administration has shown the courage to take on arms-control ideologues, but it has not been willing to take the next logical step and begin work on a space-based defense. This has disappointed many missile-defense advocates who believe that only space-based, boost-phase interceptors can knock down enough enemy missiles in the early stages of an attack to keep mid-course and terminal defense systems from being overwhelmed.

Building an effective boost-phase interceptor program would involve the further weaponization of space, and is extremely controversial. The idea upsets liberals, most foreign governments, and the transnational organizations of the "international community." But the U.S. is under no obligation to eschew space-based missile defense. The Outer Space Treaty of 1967 prohibits placing weapons of mass destruction in space, but other types of weapons are allowed. In fact, the Soviets were the first to station a weapon in space when, in the early 1970s, they equipped one of their Salyut space stations with a 23-millimeter automatic cannon. Rumor has it that they fired it once, with less-than-satisfactory results.

Some people who don't object to the idea of space-based defense in principle oppose it on practical grounds, pointing to cost overruns and delays on many of the military's main space projects. But these problems are the result of Clinton-era blunders. Under Clinton, the Defense Department disencumbered itself of considerable engineering talent and left contractors with little effective supervision. At the same time, the process used to define the capabilities required in future military spacecraft went wildly out of control. A perfect example of this was the way the U.S. Forest Service helped write the specifications for the Space Based Infrared System satellites that are designed to detect missile launches. Someone figured that, since SBIRS can detect heat from a missile launch, it might also be useful in detecting forest fires. Combining this capacity with all the other extraneous requirements made the system so complex that it will end up costing more than 150 percent of its originally estimated price, and its first components will be at least five years late reaching orbit.

The biggest obstacle to missile defense today is the lack of funds for long-term technology development. Congress has cut some missile-defense funds in the 2007 budget, particularly for a proposed European ground-based interceptor site, though these might be reinstated if the administration specifies exactly where and when it wants to build the facility. The more serious problem is that the focus of missile defense has changed. The strength of Reagan's SDI program was its willingness to develop new ideas and finance high-risk, high-payoff research. But the need to get missile defense operational as soon as possible eventually overrode the emphasis on new science and technology.

Notwithstanding its many imperfections, missile defense is now in place, and this is a step in the right direction. It provides the U.S. military with a foundation on which to build an ever-improving system. When, and if, that system is equipped with space-based interceptors, it will go a long way toward Ronald Reagan's goal of making nuclear missiles "impotent and obsolete."

Mr. Dinerman writes a weekly column for TheSpaceReview.com.