The issue:

Missile technology is proliferating. It remains unclear how quickly foes such as Iran and North Korea could develop a capability to strike the United States with missiles, but the U.S. says Iran is already capable of hitting Europe. The United States is spending nearly $10 billion a year on missile defense when military budgets are stretched. But the programs have yet to prove that they can reliably knock long-range missiles out of the sky and protect the U.S. from emerging threats.


Where they stand:

Early in his presidency, Barack Obama replaced a George W. Bush-era plan for missile defense in Europe that had roiled relations with Russia. Obama says his four-stage plan would protect Europe and the United States as foes develop more sophisticated missiles. The announcement initially eased tensions with Moscow, which considered the previous plan a threat to its nuclear might. Obama has proposed cutting missile defense spending in 2013 by about 7 percent, to $9.7 billion.

Republican rival Mitt Romney would reverse Obama's proposed cuts to the program. He wants to maintain Obama's plans in Europe — so long as they work. He argues that part of Obama's plan is based on theoretical technology and was designed as much to appease Russia as to address threats from Iran. Romney has called Russia the top geopolitical foe of the United States.


Why it matters:

Americans have long taken comfort in the distance from tension in Asia and Europe provided by two wide oceans. Intercontinental ballistic missile technology undermines that security by offering foes the ability to strike quickly from great distance with weapons of mass destruction.

Missile defense has been contentious since Ronald Reagan proposed the idea of making ICBMs obsolete in a nationally televised address in 1983. The initiative was dubbed "Star Wars." Critics say that despite about $150 billion spent since then, the U.S. is far from achieving Reagan's goal. Even supporters claim only a limited capability against long-range missiles. Recent government-commissioned reports by the National Academy of Sciences and other panels have highlighted critical problems with the effectiveness and management of the programs.

Despite those questions, both political parties largely support current missile defense programs. Romney has not proposed any major deviations from Obama's path, but could steer policy in Europe back to a more confrontational approach with Russia.

The United States maintains that missile defenses are aimed at countering attacks from rogue regimes and would be impotent against the arsenals of major nuclear powers such as Russia and China. But Moscow says even a limited capability against its ICBMs could destabilize the balance that deters the United States and Russia from contemplating nuclear confrontation. China has also increasingly raised objections to U.S. and Japanese missile defense assets in Asia.

Although Moscow initially welcomed Obama's shift in missile defense policy, Russian officials have since objected to the latter phases of his four-stage plan. A faster interceptor, still in development, is to be deployed in 2020, theoretically capable of shooting down ICBMs that can reach the U.S. Russians also worry about an increased U.S. military presence in Eastern Europe. And Russian President Vladimir Putin said Romney's identification of Russia as a top foe justifies his concern about U.S. missile defense.

Republicans wonder if the U.S. will roll back the latter stages of the plan. They cite Obama's comment in March to Dmitry Medvedev, then Russia's president, when Obama was unaware he was speaking on an open microphone. Obama told Medvedev he would have more flexibility on the issue after November's election.