It is not clear whether deterrence based on the threat of retaliation can work in a world of diverse nuclear actors.

Some experts doubted it would last even when there were only two major nuclear players, because eventually leaders would arise who were irrational or accident-prone. Today, the global landscape presents several examples of leaders who cannot be trusted to behave in ways required to maintain stable deterrence.

However, there is a different approach to dealing with ballistic missiles capable of causing mass destruction -- regardless of whether they carry nuclear, chemical or biological warheads.

It describes the benefits of intercepting ballistic missiles in the earliest, most vulnerable phase of their trajectory, known as boost phase. That is when missiles are easiest to track, and when they present the smallest number of targets that must be destroyed to negate their destructive potential.

Boost-phase interception is not an alternative to deterrence and arms control, nor is it an alternative to intercepting warheads later in their trajectory. It complements all those other approaches to nuclear security by presenting hostile countries with powerful incentives for not acquiring or launching nuclear weapons.

With regard to deterrence and arms control, boost-phase interception reduces the military value of ballistic missiles so that countries possessing them are less likely to take risks and more likely to disarm. With regard to attacking warheads later in their trajectory, boost-phase interception makes that task much easier by thinning out or entirely eliminating threats.

It is necessary to describe the ballistic-missile threat, detail U.S. government plans for countering it, and then explain why boost-phase interception offers the greatest potential for effective defense. There are already several promising approaches to achieving boost-phase interception that offer a low-cost path for moving forward.

Although current developing boost-phase technology focuses mainly on ballistic missiles armed with nuclear warheads, the concepts and programs it explores are equally applicable to ballistic missiles carrying other kinds of kill mechanisms -- such as those that threaten U.S. forward-deployed forces.

The threat posed to the United States and its allies by ballistic missiles in the hands of potentially hostile countries is complex. Ballistic missiles are built with many different ranges and carry a wide array of warheads. Some countries, such as Russia and China, are capable of using ballistic missiles to deliver high-yield nuclear warheads over intercontinental distances, and the warheads are equipped with penetration aids to confuse defenders.

Other countries have missiles of lesser range and fewer warheads with simpler features. And some countries have mastered the technologies associated with ballistic missiles but currently lack a nuclear-weapons capability.

(Part 3: The changing nature of the ballistic missile threat)

(Loren B. Thompson is chief executive officer of the Lexington Institute, an Arlington, Va.-based think tank that supports democracy and the free market.)

(United Press International's "Outside View" commentaries are written by outside contributors who specialize in a variety of important issues. The views expressed do not necessarily reflect those of United Press International. In the interests of creating an open forum, original submissions are invited.)