Vice-president-elect Joe Biden issued a prescient warning in the last days of the presidential campaign: If Barack Obama were elected president, he would be tested by a major international crisis soon after taking office. Biden was wrong about one thing: The test has come even before President-elect Obama is sworn in.

Within hours of Obama's impressive victory, another new leader, Russian President Dmitry Medvedev, warned that Russia would deploy short-range missiles capable of hitting NATO territory if the new American president goes ahead to build a missile defense system to protect Europe. It's unclear where a President Obama will come down on this issue. He's been on both sides during the campaign.

The idea of an anti-missile defense system, of course, is not new. The United States has been working on an anti-missile system to protect our territory since the Reagan administration. The Strategic Defense Initiative -- often derisively dismissed as "Star Wars" by its critics -- fundamentally changed the way the U.S. approached the idea of nuclear war.

Through much of the Cold War, the United States based its defense almost entirely on a good offense: mutually assured destruction (MAD). We would have so many weapons that the Soviets would realize that an attack on us would be suicidal. If they launched a surprise nuclear attack on us, enough of our missiles would survive to retaliate against them, and annihilation would be the fate of both sides.

But Reagan changed the equation. Essentially abandoning the 1972 Anti-Ballistic Missile Treaty, which allowed the Soviets and the U.S. to set up anti-missile systems to protect only their two capitals, Reagan announced he would explore building a defense shield to protect the entire country.

Some 20 years later, U.S. technology in this area has advanced to the point that we are capable of deploying a limited system to protect our allies. Last year, the U.S. announced that negotiations were under way with some of our friends in Europe to deploy anti-missile systems on their territory. For some of those allies, the primary threat they fear is a nuclear-armed Iran. Although, Poland, with whom we've now signed an agreement, also fears a newly belligerent Russia. But the Bush administration has been at pains to reassure an insecure Russia that any American-deployed system would be purely defensive -- a so-called "hit-to-kill" strategy in which a missile's technology would not even include explosives but would rely on intercepting a nuclear missile before it hit its target.

Russia has now made it clear to the incoming president: Move ahead with deploying 10 interceptor missiles in Poland and a radar system in the Czech Republic and we will deploy short-ranged missiles near Poland. So what will our new president do? The last time Russia flexed its considerable muscle by invading Georgia, candidate Obama at first acted as if both sides were equally to blame. He later righted himself, condemning Russia as the aggressor.

But those were just words -- after all, the only thing a candidate has at his disposal. On Jan. 20, President Obama will have to take action. As Abe Greenwald, my fellow blogger at Commentary magazine's Contentions wryly noted, "In Russia, we now witness ice-cold realism at its most intractable. This is an enemy that advances when we blink."

President-elect Obama is busy with preparations for the transition to his new office. But the Russians won't wait -- and neither will our enemies. Obama must signal that there will be no major shifts in American foreign or defense policy, irrespective of all the campaign rhetoric about change. He could do so by quickly announcing his picks for secretaries of state and defense. I doubt Colin Powell wants another term at state, but perhaps he would view defense as a new challenge. At the very least, such a choice would inform Russia that despite partisan wrangling in election years, the United States remains committed to protecting our allies and ourselves, and a President Obama has no plans to change that.