WASHINGTON – President Bush (news – web sites) on Tuesday ordered the Pentagon (news – web sites) to have ready for use within two years a bare-bones system for defending American territory, troops and allies against attack by ballistic missiles.

Defense Secretary Donald H. Rumsfeld cautioned against viewing the plan as a foolproof means of defense. He described the planned initial capability as “better than nothing” and said it would evolve in ways that incorporate technological advances, lessons learned from testing and help from allies.

If it goes as planned, the system would expand over a decade and beyond, eventually providing defense against all ranges of ballistic missiles, at every stage of their flight and from any point on the globe.

The Bush administration put no final price tag on the project, but will ask Congress to allocate $1.5 billion in 2004-05. That is on top of the roughly $8 billion a year the Pentagon already has budgeted for missile defense. The extra money would pay for additional short-, medium- and long-range missile interceptors.

Critics question whether Bush’s goal is feasible and whether the threat of attack is sufficient to justify the expense.

David Wright of the Union of Concerned Scientists said the president’s plan was unproven and illusory. He noted that a missile defense system built in the 1970s was deemed inefficient and was shut down in a matter of weeks.

“No one knows if the government will admit its mistake so quickly this time,” Wright said.

The most ambitious version of missile defense was President Reagan’s Strategic Defense Initiative of 1983. He envisioned an impenetrable shield against the Soviet Union’s arsenal of thousands of missiles. That effort, called “Star Wars” by its critics, foundered until it was killed by the Clinton administration.

Bush said his project, which has been in development for years and the subject of intense international debate, is an essential step toward providing defenses against 21st century threats. They include the possibility of terrorist groups launching ballistic missiles armed with chemical, biological or nuclear warheads.

“When I came to office, I made a commitment to transform America’s national security strategy and defense capabilities to meet the threats of the 21st century,” Bush said in a statement.

“Today I am pleased to announce we will take another important step in countering these threats by beginning to field missile defense capabilities to protect the United States as well as our friends and allies.”

President Clinton (news – web sites), in his final months in office, considered and rejected the idea of committing the United States to deployment of a ground-based missile defense system. He said he was not convinced the technology was developed enough, and he ordered the Pentagon to continue research and testing.

Since January 2001, the Pentagon has been successful in four of five attempts to intercept a long-range warhead in space with an interceptor launched from Kwajalein Atoll in the central Pacific. But last week, in the most recent test, the interceptor rocket failed to destroy a dummy warhead.

The Pentagon has succeeded in three consecutive tests of a ship-launched interceptor, the Standard Missile-3, against medium-range missiles.

Rep. Duncan Hunter (news, bio, voting record), the likely next chairman of the House Armed Services Committee, praised Bush’s decision and said Congress would likely approve the additional $1.5 billion.

David Sirota, spokesman for Democrats on the House Appropriations Committee, questioned why the president “thinks we are so flush with cash that we can afford billions to deploy a technology that might not even work.”

The Bush plan was outlined at a Pentagon news conference by Lt. Gen. Ronald Kadish, director of the Missile Defense Agency. Among the key elements: