Mr. Bush's policy, and an unpublished National Security Presidential Directive that aides say closely parallels the public document, is clearly intended to counter threats from nations like North Korea, which Mr. Bush has repeatedly charged with using its nuclear and missile programs to try "blackmail."

During the last presidential campaign, the threat that North Korean missiles could eventually reach American shores became the justification for Mr. Bush's argument for withdrawing from the 1972 Antiballistic Missile Treaty. He formally pulled out of the treaty last year.

In drafting the new policy, Mr. Bush's aides have eliminated the distinction between a "national" missile defense and defenses to aid allies, which the new policy calls an "artificial distinction."

"The defenses we will develop and deploy must be capable of not only defending the United States and our deployed forces, but also friends and allies," the document says. Mr. Bush's aides, briefing reporters today, said the timing of the release was partly linked to the arrival on Thursday of Japan's prime minister, Junichiro Koizumi, who will visit Mr. Bush at his Texas ranch.

Japan has been engaged in research on missile defenses, and the North Korean crisis has fueled political debate in Tokyo about joining in the American-led program.

In Malaysia, the Russian defense minister, Sergei B. Ivanov, repeated an earlier Russian offer to cooperate with the United States in the construction of a missile defense. But he listed a number of conditions, including "the preservation of each side's intellectual property, the demilitarization of space and total transparency regarding missile defense."

The missile defense policy put forth today is the third policy directive that the White House has issued since September, as it seeks to reorient national security policy. The first was "The National Security Strategy of the United States," best known for enshrining pre-emptive military action as a keystone of American policy after the Sept. 11, 2001, attacks.

The second called for specific action to intercept and to counter nuclear, chemical and biological weapons and their export. On the day it was formally published, Spain stopped a North Korean freighter filled with missiles headed to Yemen, but after Yemeni protests it was allowed to proceed.

There were few surprises in the new missile defense policy, but it stated in clear language some shifts in approach that have gradually become clear over time.

It notes, for example, that "The United States will not have a final, fixed missile defense architecture," but rather an "initial set of capabilities that will evolve to meet the changing threat." One official likened the plan to the Air Force, which regularly purchases new planes and deploys them around the globe.

The policy appears to treat missile defenses as another way of sowing doubt in the minds of a potential enemy. "We must devalue missiles as tools of extortion and aggression," the policy says, "undermining the confidence of our adversaries that threatening a missile attack would succeed in blackmailing us. In this way, although missile defenses are not a replacement for an offensive response capability, they are an added and critical dimension of contemporary deterrence."

There is no talk in the policy of a shield, and no standards by which to measure the effectiveness of the missiles - the issue that dominated the debate over missile defense before Mr. Bush took office.

The policy argues that by reducing, although not eliminating, the chances that a country like North Korea could threaten the United States with the launching of one or two missiles, it serves the purpose of "undermining their military utility."

The first deployments of the missiles were announced in December. Under the plan, the military would field a total of 10 ground-based interceptors in Alaska and California by sometime next year, with 10 more in Alaska the following year.