President Obama has wasted no time in trying to turn back the clock on missile defense, one of the great foreign-policy achievements of the Bush administration. If he succeeds, he will cause European allies to question America’s commitment to their security, convince Russian leaders that bullying has its rewards, and leave the United States vulnerable to nuclear blackmail by Iran. The roots of the problem lie in Obama’s mixed message about missile defense during the presidential campaign: He said missile defense was necessary because of threats from Iran and North Korea but also promised to cut funding for what he called “unproven” technologies. In one venue, he wanted to sound like a foreign-policy realist; in another, he sought to placate liberals who have despised missile defense on ideological grounds ever since Ronald Reagan proposed it. The day after Obama’s election, Russian president Dmitri Medvedev tried to exploit this perceived weakness. He warned against U.S. plans to deploy a small missile-defense system in Eastern Europe. Specifically, he said that if a NATO-approved measure moved forward he would order short-range ballistic missiles into Kaliningrad, a Russian exclave nestled between the borders of Lithuania and Poland. Moscow’s saber-rattling appears to have paid off. In February, Obama sent what the New York Times described as a “secret letter” to Medvedev. It offered the Russians a deal: If they will help prevent Iran from developing long-range missiles, Obama will pull the plug on the missile-defense system in Eastern Europe. What comes next almost could be choreographed: Russia will pledge cooperation and perhaps even support a toothless resolution at the U.N., Obama will declare the abandonment of missile defense a diplomatic imperative, and Iran will continue going about its sinister nuclear business. (Last month, the ayatollahs’ rocket scientists put a satellite into orbit, demonstrating a technological prowess with worrisome military implications.) Meanwhile, the members of NATO will wonder whatever happened to the multilateralism that Obama once vowed: They had joined in mutual support of the missile-defense system only to have the new president engage in a bit of freelance diplomacy with the Kremlin. For Moscow, the real issue isn’t missile defense. Its repeated condemnations of a radar in the Czech Republic and ten interceptors in Poland never have made much sense on their face, because the system poses virtually no deterrent to the former Soviet Union’s massive arsenal of missiles. What the Russians really want is the United States out of Eastern Europe, a sphere of influence they regard as theirs alone. Any American military presence in the region will be met with resistance. As it happens, basing the missile-defense system in Eastern Europe is no reckless provocation; defending London and Paris from Iranian missiles can’t be done from elsewhere with existing technologies. The Bush administration is not entirely blameless in the current predicament. It deserves praise for acting boldly on missile defense, first by withdrawing from the obsolete Anti-Ballistic Missile Treaty and then by deploying interceptors in Alaska and California to provide a rudimentary shield against possible North Korean aggression. Yet in Eastern Europe, where negotiations with Poland dragged on longer than they should have, it might have moved with more urgency — and put its successors, so eager to walk away from missile defense, in a tougher position. Once a missile-defense system is operational, after all, it becomes politically difficult to abandon it. The full extent of Obama’s hostility to missile defense won’t be known until the White House releases its budget proposals in a few weeks. In all likelihood, its strategy will involve a return to the Clinton years, when missile defense was chronically underfunded and the ballistic ambitions of rogue states such as Iran and North Korea were treated as inconveniences to be ignored rather than threats to be countered.