In the abstract, ballistic missile defense is an attractive idea. Ronald Reagan's hope of "rendering nuclear weapons impotent and obsolete" by means of the original Strategic Defense Initiative promised to restore the sense of invulnerability to attack on the homeland once provided by the Atlantic and Pacific Oceans. If American scientists were finally able to develop a cost-effective "missile shield" that worked, ballistic missile defense would enjoy broad support across the American political spectrum.

There is a threat. If a workable system could be fielded, it could prove effective against China's small stock of DF-5 ICBMs as well as the limited numbers of ICBMs other potential threats might develop. Yes, a functioning theater missile defense system in Alaska would not be effective against Russia's massive ICBM arsenal--but the possibility of a large-scale assault from Russia (as opposed to an accidental launch) is, in a post-Cold War world, virtually nil.

But current plans for the deployment later this year of a ballistic missile defense system will not make America safer. Because the system currently in development has not been tested under realistic conditions, we have little idea if it will work. What we do know is that it will be terrifically expensive; we further know that it remains impossible to say just what that means.  Finally - and crucially - investment in ballistic missile defense pulls scarce time, energy and money away from successful prosecution of the war on terror.

The Pentagon's testing of the system has produced limited, unconvincing successes at best. According to a non-partisan Congressional audit published in April of this year by the General Accounting Office, of the eight flight intercepts attempted so far, only five have been successful - and those have been largely "repetitive and scripted." Important system components have yet to be flight-tested together. The report's title says it all: "Actions Are Needed to Enhance Testing and Accountability."

Simply put, the successful tests - on which political support for ballistic missile defense deployment depends - have been too easy. They have failed to simulate the actual speed and altitude of an incoming missile or the sort of decoys it might use to thwart defensive countermeasures. If no one knows whether or not the system will actually intercept an incoming missile that is designed to defeat defensive counter-measures, should American taxpayers spend the tens or hundreds of billions of dollars necessary to deploy it?

Should Americans feel protected by a system that Dr. Philip E. Coyle III, a former head of weapons testing at the Pentagon, calls "no more than a scarecrow, not a real defense?" Or as the senior Democrat on the Senate Armed Services Committee, Carl Levin of Michigan, said recently, "If we want a missile defense that works rather than one that sits on the ground and soaks up money, we should not shy away from realistic testing requirements."

In July of 2000, Senator Dick Durbin (D-Ill.) offered an amendment to a Defense Authorization Bill that called for additional testing of the national missile defense plan against decoys and countermeasures and the resurrection of an independent panel to review the test results. Supporters of missile defense defeated the amendment in a 52-48 vote. 2000 was, of course, an election year, and the Senate vote fell predictably along party lines. The Pentagon's determination to deploy a missile defense system this year, before it has been properly tested, is certain to become a political football in this election year.

Some of the current missile defense plans may actually endanger our friends and allies. The Pentagon's Missile Defense Agency (MDA) is currently working on two kinds of defensive weapons capable of defeating virtually all potential counter-measures by destroying a hostile missile during the "boost phase" - its first few moments of flight. The first group consists of anti-ballistic missiles propelled into a ballistic missile at high speed. The second system is based on a set of lasers that heat up the missile until it fragments. Stopping a hostile missile in its boost phase is appealing because the missile hasn't yet had time to deploy decoys or to fragment into smaller weapons. But the boost phase lasts only three or four minutes - and therefore demands extremely fast response and is difficult to target. A study published in July 2003 by the American Physical Society (APS) found that, even in the unlikely event that either the anti-ballistic missiles or the lasers could successfully strike a missile in its boost phase, they would not destroy the hardened warhead. Thus, even a successful intercept of a ballistic missile launched from a rogue state - say, North Korea or Iran - runs the risk of dropping the missile's warhead filled with chemical, biological or nuclear weapons on Canada, Russia or western Europe.

It is not simply that we don't know whether or not the system will work.  We also don't know what it will ultimately cost. The Strategic Defense Initiative was originally intended to be a $26 billion research plan, but the Pentagon now says it has spent more than $80 billion on missile defense since 1985. The Bush administration's request for fiscal year 2005 alone tops $10 billion. In fact, missile defense projects consume more research and development dollars than any other military program.  

Further, according to the GAO report, there are significant gaps in program cost estimates provided by the Pentagon. The MDA predicts an additional $53 billion will be needed for missile defense between 2004 and 2009 but has not specified likely additional costs for operations, maintenance and other so-called lifecycle expenses. At a time when America's force commitments stretch troop strength to a critical point, a costly system on spec is not worth the investment.

Beyond questions of feasibility or cost, is ballistic missile defense the proper response to the threats the United States now faces? Which is the greater current or future threat to America's national security: a North Korean ICBM launched on Los Angeles or a terrorist attack on New York City's ports or public transportation? The best reason the US government should sharply limit its commitment of resources to ballistic missile defense is that the greatest threat to American national security won't come from a rogue state launching a ballistic missile.

President Bush has correctly identified the greatest threat to American security as the intersection of terrorism and weapons of mass destruction. But the 9/11 attacks demonstrated that terrorist groups do not need a committed state sponsor to carry out attacks on Americans and that many weapons of mass destruction are significantly cheaper than a ballistic missile. Terrorists and rogue tyrants have cheaper and less self-incriminating ways of damaging US national security. They don't need intermediate range missiles to attack the American mainland when hijacked jets, suitcase bombs, attacks on vulnerable ports or subways or a few vials of anthrax are more cost-effective and more difficult to defend against or to trace.

The tens of billions of dollars spent on ballistic missile defense system would be more usefully directed toward securing the nation's ports and systems of mass transportation; destroying existing stockpiles of chemical, biological and nuclear weapons - which might otherwise end up being smuggled to the highest bidder; and securing all enriched uranium and plutonium to prevent terrorist groups from attempting to build their own crude nuclear devices. Money saved on missile defense could be redirected toward improving human and technical intelligence capabilities, thwarting attempts to proliferate weapons and weapons technology, and increasing US troop strength at a moment when the Pentagon is hastily improvising troop support for undermanned American units battling insurgents in Iraq and Afghanistan.

To make the nation safer, policymakers must counter the most pressing threats to America's homeland security with the weapons and strategies most appropriate to defeating them. Today's US military is stretched too thin; the missions to stabilize Iraq and Afghanistan remain far too important for anything less than a robust commitment of American military and civilian resources. With some of the money that is currently devoted to ballistic missile defense, the military can and should add forty to fifty thousand soldiers to active duty. Half these men and women should be combat troops trained in counterinsurgency. The other half should be civil affairs personnel trained in civilian reconstruction. Both would be invaluable in stabilizing Iraq or Afghanistan. A sharp increase in the number of special forces personnel would also prove a wise investment.

In addition, the National Guard should be retrained to provide civil defense for the American mainland. The crucially important work of protecting America's ports, rail lines, waterways, public transportation and other vulnerable facilities could be enhanced by Guard soldiers, better suited to domestic than to overseas service.

Modest sums of money might be usefully diverted from some missile defense research and development toward the implementation of non-lethal weapons technology useful in both homeland defense and counterinsurgency combat situations. A "mobility denial weapon," capable of stopping an oncoming vehicle while it is still a safe distance away, could prevent a devastating attack.