I want to spend ten minutes today explaining why missile defense is more feasible and desirable now than it was during the cold war.

Let me begin with a little bit of history.

The missile defense debate began in America about 50 years ago with the Soviet Union's launch of the Sputnik satellite, because the rocket used to loft the satellite into orbit was also capable of carrying a nuclear warhead from Russia to America.

Before Sputnik, concern about nuclear attack focused mainly on bombers, and policymakers thought they had a solution for that threat.

Ballistic missiles were a much tougher challenge, and the wave of fear that followed Sputnik led to a series of missile defense projects with names like Nike-Zeus, Sentinel and Safeguard.

All of those early programs involved using nuclear explosions to destroy incoming Soviet warheads, because the military lacked technology for building interceptors that were more discriminate.

But in 1983 -- halfway between the Sputnik launch and today -- President Ronald Reagan proposed something truly revolutionary called the Strategic Defense Initiative.

SDI was a completely different approach to defense against missile attack, because it proposed using space-based lasers and other precision weapons to destroy incoming warheads without having to rely on the gross effects of nuclear explosions.

It was at that point that I started paying attention to the missile defense debate -- in fact, I ended up teaching a class about nuclear strategy at Georgetown University.

The Strategic Defense Initiative was a very controversial program, not only because it was complicated and costly, but also because critics alleged that it would make nuclear war more likely.

However, it wasn't hard to see why the United States might want some sort of defense against missile attack, because the Russians had about 7,000 nuclear warheads aimed at America, and most of them were on ballistic missiles.

Just one-percent of that arsenal would have been sufficient to collapse the operations of our government and economy, given the lack of any real defenses. 

President Reagan considered the nation's vulnerability to be intolerable, and tried to find an alternative in SDI.

But the search for an alternative was cut short by the collapse of communism that began towards the end of Reagan's tenure.

Once the Berlin Wall came down, funding for missile defense was reduced and concern about nuclear attack receded.

That seemed like a sensible response to the declining danger,  but in retrospect you could have made the opposite case -- that the nuclear threat was shrinking to manageable proportions, and missile defense had finally become a practical possibility.

The Clinton Administration thought otherwise -- at least until North Korea began launching its own ballistic missiles in the late 1990s.

At that point, a bipartisan consensus began to emerge that some sort of regional defense against ballistic missiles was needed.

President Bush subsequently expanded the Clinton plan to encompass near-term deployment of a modest national missile defense system, abandoning the ABM Treaty as a relic of the cold war.

Which brings us to where we are today, with a theater missile defense program that enjoys broad support in Congress and a national missile defense program that provokes only muted opposition compared with the fierce battles of the Reagan era.

What we see in this brief chronology of missile defense developments is that there has been a gradual convergence of views between the two national parties on missile defense as threats have changed, costs have decreased and technologies have advanced.

Against that backdrop, I'd like to use the balance of my time to explain the flaws in our offensively-based strategy for nuclear security, and explain why conditions today make a defensive alternative more workable and necessary.  

The Problem With Deterrence

Many Americans do not realize that our nation's nuclear strategy for the last two generations has been based on a doctrine of assured destruction that makes vulnerability a virtue.

It is very different from the strategies of the past, and policymakers did not embrace it because they thought it was the most appealing approach to global security.

Rather, they devised the doctrine of assured destruction because they couldn't figure out any other way of stabilizing superpower relations in an era when leaders would have unlimited destructive power at their fingertips.

In the past, the main goal of strategy was to protect the nation by defeating attackers, but when a single bomb can destroy an entire city and each side has thousands, that becomes nearly impossible to do.

Even if your defenses are 99% effective -- an unprecedented level of performance -- you can't prevent a determined attacker from destroying the nation.

So how do you avert catastrophe, knowing that your own nuclear arsenal gives the other side a strong incentive to attack if they think they can disarm you in a first strike?

The doctrine of assured destruction formulated in the early postwar years and refined in the 1960s argues the best way to protect the nation is to make it inescapably clear to any enemy that a nuclear attack on America will be suicidal.

In other words, no matter how much firepower the enemy uses in a surprise attack, enough of our arsenal will survive to wipe out the attacking nation.

The enemy is thus deterred from attacking by the unavoidable consequences of his own actions.

But defense secretary Robert McNamara took this logic one step further, concluding that in order to maintain a stable deterrent relationship, both sides had to be equally vulnerable to the other's nuclear weapons, so neither would feel insecure about a surprise attack.

Thus his strategy came to be known not just as assured destruction, but mutual assured destruction -- MAD, to use the acronym favored by critics.

It certainly sounds "mad" when compared with more traditional strategies, but McNamara and the presidents he served faced a practical problem that they couldn't come up with an alternative capable of defeating the Russian nuclear threat and thus protecting the nation more effectively.

So they settled for creating a situation in which any rational enemy would view attacking America as an act of suicide.

Two decades later President Reagan labeled the whole concept of mutual assured destruction immoral, and launched his search for an alternative.

But that alternative never fully materialized, at least for dealing with our most potent adversaries, and so even today the nation's survival rests on a foundation of deterrence rather than real defense.

The reason many Democrats opposed missile defense after the 1960s was because they thought it would destabilize the balance of terror that results from each side having an assured retaliatory capability -- provoking an arms race as the two sides sought to bolster their deterrent forces.

That's a legitimate fear within the framework of assumptions supporting assured destruction, but there are some fundamental flaws in the strategy...

--  First, it assumes that adversaries armed with nuclear weapons will be rational.

--  Second, it assumes they will correctly interpret the signals we send them.

--  Third, it assumes they will not find some way of disarming us in a surprise attack.

--  Fourth, it assumes that none of the parties will make mistakes such as launching weapons by accident.

--  And fifth, it assumes a relatively simple world in which there are only a few nuclear players to keep track of.

If we cannot assume rational adversaries, or clear communications, or the impossibility of a successful first strike, or careful handling of weapons, or a small number of nuclear powers -- well then, the strategy begins to break down.  

Unfortunately, all of those defects in the theory of assured destruction seem to have become more relevant in the real world since the cold war ended, calling into question its efficacy as a continuing source of global security.

The world has become a much more complicated place since the theory of mutual assured destruction was formalized in the 1960s, and our ideas about security in the nuclear age may be out of step with current geopolitical trends.

Even in its heyday we couldn't really prove deterrence was working, because we couldn't read the minds of our adversaries.

Today we face a wider range of enemies, some of whom aren't even countries, and their thought processes are much more diverse. 

Missile Defense Makes Sense Today

That brings me to the crux of my argument for today, which is that missile defense is both more feasible and more desirable now than it used to be.

Missile defense is more feasible because the nuclear threats that elicit greatest concern today are less challenging than in the past, and the defensive technology we possess for dealing with those threats has advanced considerably.

Missile defense is more desirable, because the universe of potential adversaries possessing nuclear weapons is growing increasingly diverse, undercutting the viability of the assured destruction approach to national security.

In other words, our ability to build missile defenses that work appears to be increasing, while our ability to deter nuclear attack by relying solely on the threat of retaliation appears to be decreasing.

Let's look at the first part of that argument, the feasibility dimension, first.

When President Reagan proposed the Strategic Defense Initiative 25 years ago, our main nuclear adversary -- the Soviet Union -- had 7,000 nuclear warheads capable of hitting the United States and thousands more capable of hitting our allies.

SDI had to be designed using futuristic technologies, because nothing then available to the military could conceivably cope with such a huge nuclear threat.

Today, our fears of nuclear aggression focus mainly on fledgling actors like North Korea, Pakistan and Iran who are unlikely to acquire more than a few dozen warheads and correspondingly modest means of delivery.

China and Russia continue to possess capable strategic nuclear forces that could overwhelm the defensive systems we plan to deploy, but the ideological frictions that once shaped our relations with those countries have largely dissipated.

So our worries about Chinese and Russian nuclear forces are centered largely on the danger of an accidental launch or technology leakage, threats for which a thin defensive network of the kind we are building is well-suited.

In addition to the diminished scale of the nuclear threats we face, the defensive technologies we possess have improved by leaps and bounds.

Nothing available to the Reagan Administration remotely approached the agility of the Kinetic Energy Interceptor or the sensitivity of the Space Tracking and Surveillance System.

Our sensors and networks and kill mechanisms have progressed a dozen generations beyond SDI technology due to the information revolution, providing the potential for much more lethal and cost-effective defenses.

Successful interceptions of ballistic missiles in weapons tests have gone from being rare events to commonplace occurrences, and we are deploying a panoply of new defensive systems like Standard Missile Three and Patriot Advanced Capability Three that have a high likelihood of successful engagements in the real world.

Furthermore, we on the verge of demonstrating the operational viability of laser weapons for interception of theater and strategic missiles, an idea that was little more than a dream in Reagan's day.

Thus, partly because the offensive threat has changed and partly because the defensive options have changed, missile defense is far more feasible now than it used to be.

But there is another, equally compelling reason for pursuing missile defenses vigorously in the years ahead, which is that the character and psychology of our adversaries has changed.

During the last century, the American military was preoccupied with deterring or defeating other industrial powers, and most of those powers shared a similar western heritage.

Most of the emerging nuclear actors of today are neither western nor industrialized, and our grasp of what drives their behavior is not good.

No doubt we are just as much of a mystery to them, although both sides profess to understand the evil intentions of the other.

These are not favorable circumstances in which to pursue the delicate balancing act required by assured destruction, wherein we deliberately leave our population hostage to the presumed sensibility of our adversaries.

There are simply too many opportunities for mistakes, misunderstandings, accidents or outright craziness as the nuclear club opens its doors to an increasingly diverse membership.

All the flaws that were inherent in the assured destruction strategy at its inception are magnified today because we simply don't understand our adversaries as well as we once did.

Thus missile defense isn't just more practical and feasible than it used to be it, it is also more necessary.

In fact, it may be the closest thing we have to a guarantee of national survival as weapons of mass destruction continue to fall in the hands of movements and actors whose motivations can only be guessed.