WASHINGTON - After nearly two decades of national debate over the wisdom and utility of trying to intercept missiles fired at the United States, President Bush ordered the Pentagon yesterday to field within two years a modest antimissile system.

Bush's decision marked a major turning point in a debate that has consumed Washington and defense organizations since Ronald Reagan first announced a far more ambitious space-based missile shield.

A year ago, Bush withdrew from a treaty signed in 1972 with the Soviet Union that banned such systems; his action yesterday was the first time the United States had actually moved to field such a system, even though its capabilities are far more limited than proponents once hoped and its reliability still in doubt.

Congress must still approve $1.5 billion in additional financing over the next two years to field the administration's bare-bones system. Since Bush took office, Congress has approved about $8 billion a year for research in missile defenses.

The announcement came as the United States is once again in a tense standoff with North Korea, whose missile program and nuclear ambitions reinvigorated the drive for missile defenses in the late 1990s and became a centerpiece of Bush's campaign for president.

Yesterday, Bush was careful not to oversell the amount of protection it could provide against an attack, and administration officials acknowledged that the initial system would be easily overwhelmed by any attack from Russia or China.

"While modest, these capabilities will add to America's security and serve as a starting point for improved and expanded capabilities," Bush said.

Secretary of Defense Donald H. Rumsfeld, who led an independent panel in 1998 that described a growing missile threat from North Korea, Iraq and Iran, called yesterday's approach "a start" and "better than nothing."

Under the plan, the military would field a total of 10 ground-based interceptors in Alaska and California that by 2004 would be able to defend the United States against strikes from a few long-range missiles. Ten more interceptors would be fielded in Alaska in 2005.

The decision comes six months after the United States formally withdrew from the Anti-Ballistic Missile Treaty of 1972. That withdrawal lifted constraints on the Pentagon to test and field a system to shoot down missiles.

The initial system is designed to give the United States a limited defense against missiles launched accidentally or by nations such as North Korea - and perhaps eventually Iraq and Iran.

While the administration said it was a coincidence, the timing of yesterday's decision coincides with threats by North Korea to resume production of plutonium at its main nuclear site and perhaps to conduct new tests of its long-range Taepo Dong missile.

In deciding to announce the plan yesterday, the administration is essentially short-circuiting the debate over whether the system works, just as its withdrawal from the treaty short-circuited whether the United States could legally do it.

It would also allow Bush to fulfill his campaign pledge to field the start of an anti-missile system during his first term.

Bush and Rumsfeld warned against expecting a foolproof system at first, saying it would evolve into a more sophisticated one as scientists and engineers incorporated technological advances and lessons learned from testing.

Rumsfeld said: "I wouldn't want to overplay it. I wouldn't want to oversell it."

The action that Bush ordered caps nearly two decades of highly contentious debate over the size, scope and cost of defending the United States against a missile attack.

The most ambitious version of missile defense was Reagan's Strategic Defense Initiative of 1983. He envisioned a space-based impenetrable shield against the Soviet Union's arsenal of thousands of missiles. That effort, called "star wars" by its critics, was killed by the Clinton administration.

Even the program's staunchest supporters acknowledged yesterday that the Pentagon was fielding a research-and-development program that had many kinks to work out.

For example, the radar for the system has not been built, and the booster rockets that lift the interceptors into space are unreliable. Pentagon officials said they would upgrade existing radars for now and try to work out problems with the booster rockets by next fall.

"President Bush's announcement today that he plans to deploy missile defense systems starting in 2004 violates common sense by determining to deploy systems before they have been tested and shown to work," said Sen. Carl Levin, a Michigan Democrat and chairman of the Armed Services Committee.

But supporters of the administration's plan said fielding even a limited system was necessary as several potentially hostile countries continue to develop missile programs.

"The case for deploying a national missile defense system has never been more clear," said Rep. Duncan Hunter, a California Republican who is vice chairman of the House Armed Services Committee. "Today, the United States cannot stop a single ballistic missile headed for an American city. The consequences of such an attack would be devastating, and the danger continues to grow."

In addition to congressional support, the administration would need approval from Britain to upgrade a long-range radar system at the Royal Air Force base at Fylingdales and from Denmark to upgrade a similar radar complex at Thule Air Base in Greenland, a semiautonomous Danish territory.

Secretary of State Colin L. Powell has been holding discussions with British and Danish ministers, and U.S. officials said they expect agreements to be reached. The British and Danish radars would provide early warning of a missile attack against the eastern United States from the Middle East.

Lt. Gen. Ronald Kadish, director of the Missile Defense Agency, said Pentagon officials have confidence that a limited working system can be put in place by 2004, even though the latest test of the ground-based system failed last week. Five out of eight tests of the ground-based system have succeeded, he said. "Test, fix, test, fix, that's what we're doing," Kadish said.