The aim of developing a missile shield is at the core of President Bush's defense policy, although the terrorist attacks of Sept. 11, 2001, showed that an enemy does not need rocket science to penetrate U.S. defenses.
After high-level meetings in Moscow last week, the Russians remained opposed to the newest twist in U.S. missile defense - extending the network of missile interceptors and radars to central Europe. But there also were strong signs that Moscow is now resigned to living with it on its doorstep.
This week a Russian delegation is in Washington to hold follow-up talks with officials from the Pentagon and State Department - a further indication that Moscow is taking a less confrontational approach.
Less clear are answers to other key questions: Will the next U.S. president keep the project on track? And, if the system eventually is completed, will it work in the event of a real attack by long-range missiles?
Of the three leading presidential candidates, Sen. John McCain is a clear supporter of missile defense. He has described it as critical to protection of the United States from adversaries like North Korea and Iran, and as a "hedge against potential threats" from Russia and China.
Sen. Barack Obama has spoken skeptically of missile defense as developed during the Bush administration, saying it requires much more vigorous testing to ensure that it would work and be cost-effective. He has not said he would stop the planned European sites in Poland and the Czech Republic, but he has questioned the timing.
"If we can responsibly deploy missile defenses that would protect us and our allies, we should - but only when the system works," Obama said last summer.
Sen. Hillary Rodham Clinton has questioned the technological progress.
"Senator Clinton has expressed concern about the Bush administration's expenditure of billions of dollars on an unproven ballistic missile defense system that has not been adequately tested or proven to work," said Clinton spokesman Philippe Reines.
History has shown that a change of administrations in Washington can have a profound effect on missile defense. Reagan's speech outlining the strategy came on March 23, 1983. When President Bill Clinton was elected in 1992 his first defense secretary, Les Aspin, quickly dismantled the program, declaring that he was taking the "star" out of "Star Wars."
Shortly before he left office, Clinton decided against deploying an initial system of missile defense. President Bush reversed course, vowing to build effective missile defenses and opting out of the 1972 Anti-Ballistic Missile Treaty in order to escape legal prohibitions on more expansive testing.
Under Bush the budget for missile defense soared. The budget he inherited upon taking office in 2001 had $4.8 billion for missile defense. The next year it jumped to $7.8 billion; this year it is nearly $10 billion. Over the course of Mr. Bush's eight years as president the cumulative total likely will hit $70 billion. That compares with missile defense budgets totaling about $36 billion over the prior 10 years.
A central point of debate for decades has been whether missile defense would work. In a sense, it is not possible to know at this stage because it has never been used against a hostile long-range missile.
On the other hand, supporters point out that it has worked in real attacks by shorter-range missiles - as in Iraq at the outset of the current war - but never those aimed at U.S. soil. Supporters also point to successful tests, while acknowledging that those simulate fairly simple attacks by single warheads rather than a possible real-world assault by multiple warheads using decoy methods.
The Union of Concerned Scientists, a longtime opponent of missile defense, argues that any country capable of building long-range missiles would also know how to use decoys and other means of fooling U.S. interceptors.
"There is little or no prospect that the United States will develop a defense system that could defend against real-world, long-range missiles in the foreseeable future," the organization said this month in a critique timed to coincide with the 25th anniversary of Reagan's "Star Wars" speech.
Among those most intimately familiar with the project, retired Air Force Lt. Gen. Ronald Kadish is a firm believer. He was director of the Pentagon's Missile Defense Agency from 1999 to 2004.
"I have every confidence that faced with the basic knowledge of what we've accomplished and how it works and the need for it, people would be willing to continue with it," Kadish said in a recent interview. He quickly added with a small chuckle, "Now, I'm an eternal optimist - otherwise I wouldn't have been in the missile defense business."
The program may have gained some credibility when an interceptor fired from a Navy ship in late February smashed into a failing U.S. spy satellite in low Earth orbit - an apparent deadeye success. Hitting a large satellite passing in orbit on a predictable path is different from hitting a missile in flight, but the fact that it was deemed a success probably lent some luster to missile defense.
Compared to the enormously ambitious goal of Reagan's "Star Wars" proposal, which many decried as pie-in-the-sky, the current missile defense program has shrunk in its goals, if not its cost. Reagan foresaw a space-based shield that would be so effective as to render nuclear missiles obsolete.
What has evolved since then is something simpler: a ground-based network of interceptor missiles, radars and communications sites that could defend against attack by just a couple of missiles at a time. The mere existence of such a system - whatever its proven reliability - might make it less likely that an unfriendly country like North Korea would even attempt an attack, supporters say.
The first two main sites to be constructed were at Fort Greely, Alaska, where missile interceptors sit in underground silos, and at Vandenberg Air Force Base, California, where a second interceptor base was established. They are linked to radars and satellites through communications systems at Colorado Springs, Colorado.
The proposed third site would be a combination of 10 interceptors based in Poland and a tracking radar in the Czech Republic. The Bush administration is negotiating with those two governments on terms for such an arrangement; the Russians are opposed but have failed to block it.