The incoming Bush administration risks making an early mistake if it rushes to build a national missile defense. A hasty move in this area could quickly deplete the good will generally accorded a new president by foreign leaders, especially those of Russia, China and Washington's main European allies.

George W. Bush should instead expand research and testing to determine what kind of defensive shield can best meet America's security needs.

A reliable antimissile system could protect the country against the future threat of nuclear missile attack from unpredictable nations like North Korea, Iraq and Iran. American intelligence agencies predict that North Korea could have the capacity to launch a handful of nuclear-tipped long-range missiles within five years and that Iraq and Iran could reach that point within a decade.

But no workable shield now exists. The prototype interceptor missile developed by the Clinton administration has so far proved highly unreliable in tests. Mr. Bush and his advisers made clear during the presidential campaign that they considered the Clinton system flawed and inadequate. They promised to consider a variety of other technologies, including sea-based and space-based systems as well as the current land-based model.

Any of those alternative approaches would require rigorous study and testing before construction commences. While that evaluation proceeds, Mr. Bush's new foreign policy team should try to persuade skeptical countries that a limited defensive system can be built without wrecking existing arms control treaties or setting off a destructive new arms race.

Their biggest hurdle will be overcoming Russia's current refusal to modify the 1972 Antiballistic Missile Treaty to permit limited national missile defenses. The ABM treaty has been a keystone of the arms control efforts of the last three decades. If America abruptly withdraws from that treaty to build a defensive system, other agreements might begin to unravel, including the two primary nuclear arms reduction treaties signed by Mr. Bush's father at the end of the cold war.

Those two treaties provide for a two-thirds reduction in both sides' nuclear arsenals from their mid-1980's peak and for a total elimination of Russia's land-based, multiple-warhead missiles, Moscow's most dangerous weapons. Already progress in carrying out the second of these treaties has been held up by disputes over missile defense rules.

China fears that even a limited United States missile shield might be able to deflect Beijing's small force of long-range nuclear missiles. In response, China, which is not bound by any nuclear arms limitation agreement with Washington, could be tempted to build hundreds of new intercontinental missiles. America's European allies do not wish to see the revival of a costly arms race.

Mr. Bush's foreign policy advisers have been around Washington long enough to know that few initial steps would be more divisive abroad than a decision to move ahead with installation of a missile defense system. Colin Powell, the prospective secretary of state, and Condoleezza Rice, the future national security adviser, also recognize that construction of even a limited system would cost tens of billions of dollars. Until the technology is perfected, there is no point in incurring these diplomatic and financial costs.