The long-running saga of the United States missile defense program began with President Ronald Reagan's expansive vision of a space-based antimissile shield. The Pentagon had explored the feasibility of interceptor missiles designed to seek out and collide with enemy missile warheads as far back as the 1940s. But Mr. Reagan's Strategic Defense Initiative, popularly known as "Star Wars", became the signature program for missile defense.

Star Wars faded into the realm of misbegotten high-tech dreams, but the idea lived on. President George W. Bush put a new focus on missile defense when he took office in 2001. The new system relied on agile but fairly ordinary rockets to smash incoming warheads rather than nuclear-powered lasers in space. Pentagon planners saw the system as a bulwark against the ultimate calamity, a nuclear attack, while skeptics ridiculed it as a defense that will not work against a threat that does not exist. The Bush administration's plans, which included facilities in Poland and the Czech Republic, increased tension with Russia. Critics were also not convinced that it was the best defense against the kinds of military threat faced by the United States in the present day. On Sept. 17, 2009 President Obama said he would scrap the plans in favor of a system focused on intercepting shorter range Iranian missiles.

THE STRATEGIC DEFENSE INITIATIVE

On March 23, 1983, Mr. Reagan announced plans for a system of exotic, space-based defenses that would make nuclear weapons impotent and obsolete. Former Defense Secretary James R. Schlesinger said the program, the Strategic Defense Initiative, which came to be called ''Star Wars,'' was nothing but ''a collection of technical experiments and distant hopes.'' But the president, Mr. Schlesinger said, treated it ''as if it were already a reality.''

Nevertheless, minutes of Politburo meetings that have come to light, show that Mikhail S. Gorbachev was, in the words of a Russian scholar, ''obsessed'' by the proposal, which he feared would lead to a new and more dangerous round in the arms race.

Hardliners in the Reagan administration believed that heightened military spending would cause the collapse of the Soviet Union.

Just when it began to look as if Mr. Reagan would be the first president in two decades to fail to get any arms agreement with the Soviets, it was announced that he and Mr. Gorbachev would meet in Reykjavik, Iceland, on Oct. 11 and 12, 1986.

There, Mr. Reagan proposed the elimination of all ballistic missiles by 1996. Mr. Gorbachev, not to be outdone, proposed the elimination of all strategic nuclear weapons, a proposal that, to the consternation of his aides, Mr. Reagan accepted.

In February 1987 Mr. Gorbachev announced the Soviet Union's willingness to sign ''without delay'' an agreement to eliminate Soviet and American medium-range missiles in Europe within five years.

President George H.W. Bush announced in 1989 that he would vigorously pursue the Strategic Defense Initiative, and later shifted the program's mission to protect against limited ballistic missile strikes. Later that year, Mr. Bush signed the Missile Defense Act, which charged the Defense Department to develop a missile defense system by 1996. The Clinton administration announced a reoriented missile defense program in 1996, deferring portions to 2000.

THE BUSH YEARS

President Bush made the missile defense program a top priority soon after taking office and cleared the way for antimissile deployments by withdrawing from the Antiballistic Missile Treaty with Russia. Among warfare experts, the administration's actions revived the type of bitter debate that began in the cold war, culminating in an antiballistic missile treaty.

Nine interceptors were installed at Fort Greely, Alaska, and two at Vandenberg Air Force Base in California as part of a broader, multilayered system planned by the Pentagon. An interceptor consists of a rocket that carries a 155-pound "kill vehicle." The Fort Greely and Vandenberg sites are primarily oriented against potential missile threats from North Korea.

In 2006 Mr. Bush proposed a system envisioned stationing 10 interceptor missiles in Poland and a sophisticated radar facility in the Czech Republic to defend against potential ballistic missile threats from Iran or other hostile nations.

The overall cost of establishing a modest ballistic missile system in Europe would exceed $4 billion through 2015, according to a G.A.O. report published in August 2009. The proposal was justified on the grounds that it would protect Europe and the eastern coast of the United States against any possible missile attacks from Iran.

The installation of 10 interceptors in Eastern Europe would have had no significant ability to defend against Russia's sizable nuclear arsenal. But the Polish and Czech governments saw the presence of American military personnel based permanently in their countries as a protection against Russia. Moscow strongly opposed the shield and claimed it was targeted against Russia and undermined national security. The United States repeatedly denied such claims.

A REVERSAL BY THE OBAMA ADMINISTRATION

President Obama decided not to deploy a sophisticated radar system in the Czech Republic or 10 ground-based interceptors in Poland, as Mr. Bush had planned. Instead, the new system his administration is developing would deploy smaller SM-3 missiles, at first aboard ships and later probably either in southern Europe or Turkey, those familiar with the plans said.

Mr. Obama's move amounted to one of the biggest national security reversals by the new administration, one that will upset Czech and Polish allies and possibly please Russia, which adamantly objected to the Bush plan. But Obama administration officials stressed that they were not abandoning missile defense, only redesigning it to meet the more immediate Iranian threat.

The Obama team relied heavily on research by a Stanford University physicist, Dean Wilkening, who presented the government with research in 2009 arguing that Poland and the Czech Republic were not the most effective places to station a missile defense system against the most likely Iranian threat. Instead, he said, more optimal places to station missiles and radar systems would be in Turkey or the Balkans.

Word of the administration's decision leaked before an official announcement was made, making for unfortunate timing, as the news came on the 70th anniversary of the Soviet invasion of Poland at the start of World War II, a date fraught with sensitivity for Poles who viewed the Bush missile defense system as a political security blanket against Russia.