Now, in choosing Mr. Rumsfeld to be his secretary of defense, President-elect George W. Bush has signaled that the politically and diplomatically divisive goal of building a shield against nuclear missiles will be at the core of the new administration's national security agenda.

In 1998, Mr. Rumsfeld, the former Republican congressman, former ambassador to NATO and former secretary of defense, oversaw a commission that concluded that "rogue" nations could threaten the United States with ballistic missiles sooner than analysts had predicted.

Conservatives who supported a missile shield hailed the findings as refreshingly candid and worrisome. Liberals who supported arms control criticized them as too focused on potential threats and not on the diplomatic and financial obstacles to building a missile shield.

Either way, the commission's report - and a provocative North Korean missile test a month later - led the Clinton administration to propose its own limited version of a national missile defense. What turned out to be one of the most influential documents in modern American military planning bears Mr. Rumsfeld's name.

"The Rumsfeld Report was the main reason the debate was gradually turned around and the administration turned around," said Senator Jon Kyl, a Republican of Arizona and an ardent advocate of a missile defense.

In his campaign for president, Mr. Bush advocated building a more expansive defensive system than the one President Clinton proposed and, last summer, deferred. Mr. Rumsfeld offered little detail, except to say he would not rule out defenses based on the ground, as Mr. Clinton proposed, or at sea and in space.

Today, Mr. Bush was no more specific, saying only that he would expect Mr. Rumsfeld to work closely with his budget director "to make sure that the missile defense receives the priority we think it must receive in future Pentagon budgets."

Still, it is clear that Mr. Bush's selection of Mr. Rumsfeld completes a national security team - including the next vice president, Dick Cheney, and the next secretary of state, Gen. Colin L. Powell - that shares the dream of building the sort of shield against nuclear missiles that President Ronald Reagan envisioned.

When his selection was announced on Dec. 16, General Powell made the case forcefully, calling a defensive shield "an essential part" of the nation's security. Mr. Bush himself referred to Mr. Rumsfeld's prominence on the issue of missile defense, citing his work as chairman of the commission, to which Congressional Republicans had appointed him.

"In picking Don Rumsfeld, we'll have a person who is thoughtful and considerate and wise on the subject of missile defense," Mr. Bush said.

Mr. Rumsfeld's report, released in an unclassified form in July 1998, was striking in its contradiction of previous analyses by the nation's intelligence agencies, which had concluded that no new nation could strike the United States with ballistic missiles for at least a decade.

Instead, the commission warned that countries like North Korea, Iran and Iraq could develop a missile "with little or no warning" - and essentially at any moment.

Since the report became public, North Korea, in particular, has undergone significant changes. Its once hermitic leader, Kim Jong Il, had been negotiating with the Clinton administration to halt its production of long-range missiles, though the White House announced today that progress had not been enough to warrant a presidential trip to North Korea to seal a deal.

Mr. Rumsfeld did not address North Korea today, but his remarks indicated that his assessment of the threat of a ballistic missile attack on America had not changed.

"There is no question but that the proliferation of weapons of mass destruction and the delivery systems for them is extensive across the world," he said.

Mr. Bush's proposals for a missile defense will face the same hurdles as President Clinton's. And since Mr. Bush's would go further, they may be more contentious, especially in Russia and China, which view missile defenses as destabilizing.

President Clinton decided against moving ahead with a limited system that began with 100 interceptors after failing to persuade the Russians to amend the Anti-Ballistic Missile Treaty of 1972.

Although Mr. Bush and Mr. Rumsfeld have not advocated abandoning the treaty, they have suggested that they will not be bound by its prohibition on developing a missile defense. Other Republicans have called for its abolition, which even the nation's staunchest European allies oppose.

Daryl Kimball, executive director of the Coalition to Reduce Nuclear Dangers, an arms control advocacy group, said the new administration would have to prove that a shield against missiles was feasible. Despite spending $4.8 billion this year, the Pentagon has had tests of a limited system fail.

"It's not only technically difficult and expensive," Mr. Kimball said. "It's a political hot potato. This is going to be much more difficult than they think."