In an age when weapons of mass destruction can be slipped into the United States in a cargo container or even a suitcase, is Ronald Reagan's 1983 dream of building an umbrella against long-range enemy missiles passé? Or is it a necessary screen against the possibility of North Korea or another rogue state tossing a nuclear-tipped rocket our way?
As the US moves ahead with testing and deployment of the system, new questions are swirling about the merits of pursuing such a costly program in a time of war and increased demand for defense dollars.
The debate comes amid enduring skepticism about the technological feasibility of erecting an effective shield. In December, the US missile defense program suffered another test failure when the rocket carrying the "kill vehicle" meant to destroy an incoming mock enemy warhead shut down before launch from the Kwajalein Atoll in the Pacific Ocean.
Pentagon officials, who attributed it to an "unknown anomaly," downplayed the failure. But critics point out it is part of a pattern - four of the system's nine major tests have been unsuccessful.
Yet the Bush administration doggedly keeps pushing the program. The administration has steadily increased funding for missile defense, although the Pentagon may trim that in coming years in order to pay for the expensive ground wars in Afghanistan and Iraq. And it's pushed ahead with deployment in Alaska and California even though some experts say key components are far from finished.
Early in his first term, Mr. Bush also pulled the US out of the 1972 Antiballistic Missile Treaty. The ABM Treaty restricted such defenses on the theory that this would slow down the nuclear arms race by maintaining a kind of standoff known as "mutual assured destruction."
The geopolitical landscape and the threat environment are very different from when Mr. Reagan launched his Strategic Defense Initiative (quickly dubbed "star wars"), startling allied nations as well as potential adversaries.
"Missile defense is gradually outgrowing the ideological disputes of the past," says national security analyst Loren Thompson of the Lexington Institute in Arlington, Va. The threat has become more diverse and less predictable, he says. At the same time, he adds, having to deal with a handful of relatively primitive weapons from a few rogue states, rather than masses of multiwarhead missiles from the former Soviet Union, means "this mission looks doable."
"The scale of likely threats is so modest that even the thin defense being built in Alaska and California may be sufficient to dissuade some problem-states from pursuing long-range ballistic missiles," says Dr. Thompson.
Others see an even greater need.
"China has developed a whole new generation of mobile ICBMs capable of hitting the US," says Baker Spring, a national security analyst at the Heritage Foundation in Washington. "And hostile governments, such as North Korea and Iran, continue to develop and produce ballistic missiles capable of inflicting real damage upon American soil."
Failure to counter such threats militarily, some advocates say, could undercut this country's military and diplomatic position in a dangerous world. Sen. Jon Kyl (R) of Arizona, a leading congressional proponent of missile defense, warns of "blackmail intended to freeze us into inaction by the very threat of missile attack."
No one underestimates the difficulty of "hitting a bullet with a bullet," which describes the technological challenge of missile defense, especially when split-second decisions need to be made during the fog of war.
As the invasion to oust the regime of Saddam Hussein got under way in March 2003, shorter-range US Patriot missiles nailed several Iraqi rockets headed for advancing coalition forces. But Patriots also mistakenly shot down a US Navy jet and a British fighter, killing the allied pilots. And countering ICBMs, which can reach US targets from halfway around the world in just 30 minutes, is far tougher - especially if an attacker also launches a bunch of decoys, as expected.
Critics liken the rush to deploy expensive ballistic missile defenses at a time when the threat as well as technology is changing to rewriting architectural plans in the middle of building a house. And they say the nine tests so far (five of which succeeded) were set up - "rigged," some say - to virtually assure success.
"A system is being deployed that certainly doesn't have any credible capability," says retired Air Force Gen. Eugene Habiger, former head of the Strategic Command, which includes all US nuclear forces. "I cannot recall any military system being deployed in such a manner."
Other critics point out that the system being deployed in Alaska and California lacks certain crucial elements, including the necessary radar, the proper satellite constellations, and the ability of the "kill vehicle" interceptors to discriminate between potential targets at a closure rate of more than 15,000 miles per hour.
"This is like deploying a new military jet fighter with no wings, no tail, and no landing gear," says Philip Coyle, the Pentagon's former head of weapons testing, now an adviser to the private Center for Defense Information in Washington.
Whether the administration's relative optimism about the potential for a US missile-defense system will continue to apply to this country's old communist adversary is another matter. Russian military officials announced earlier this year that they are developing a "revolutionary" intercontinental weapon. Rather than being ballistic (unpowered once it's left the booster rocket), this new type of warhead would be powered by a supersonic combustion ramjet allowing it to maneuver to the target.
This design may be as untested as the more fanciful aspects of star wars, such as space-based lasers.
But even proponents of ballistic missile defense say there are other things to worry about. The Bush administration, says missile defense supporter Thompson at the Lexington Institute, "has been nearly blind" to the danger of low-flying cruise missiles proliferating around the world. According to the Pentagon, nine countries will be producing land-attack cruise missiles over the next 10 years, many of them for export.
"The nation needs a balanced defensive posture, which means taking the growing cruise-missile danger seriously, even as we move to counter the more visible threat of ballistic missiles," says Thompson.