On July 3 in Krakow, a city in south Poland, the United States and Poland signed an amendment to a 2008 agreement on the deployment of ballistic missile interceptors in Poland.

"This agreement marks an important step in our countries' efforts to protect our NATO allies from the threat posed by the proliferation of ballistic missiles and weapons of mass destruction," said a joint statement issued by U.S. Secretary of State Hillary Clinton and Poland's Foreign Minister Radoslaw Sikorski.

It was the first agreement under the United States' new plan on missile defense system deployment.

From Bush to Obama

DEAL! U.S. Secretary of State Hillary Clinton exchanges documents with Polish Foreign Minister Radoslaw Sikorski at a July 3 signing ceremony for an amended missile defense agreement in Krakow (XINHUA/AFP)

After taking office in early 2001, former U.S. President George W. Bush sought to establish an integrated missile defense system. To that end, he combined the National Missile Defense and the Theater Missile Defense.

On May 1, 2001, Bush declared a withdrawal from the Anti-Ballistic Missile Treaty, paving the way for the deployment of a national missile defense system. The United States accelerated the deployment of the system, following the September 11 terrorist attacks in the same year.

In January 2007, with the aim of preventing missile attacks from Iran and North Korea, the Bush administration proposed deploying missile defense systems in Poland and the Czech Republic. Negotiations began around the same time.

A year later, in July and August of 2008, the United States signed agreements with the two countries respectively, allowing for the deployment of a radar site in the Czech Republic and 10 missile interceptors in Poland.

After Barack Obama took office in 2009, he ordered the Department of Defense and the Joint Chiefs of Staff to reevaluate the U.S. missile defense system. They came to two conclusions.

First was that Iran's experiments with short- and medium-range missiles went faster than the United States expected, while its experiments with intercontinental ballistic missiles went slower than the United States expected. In other words, in the short term, Iran's missiles would not pose a direct threat to the United States, but could be a major threat to its European allies.

In June 2010, U.S. Secretary of Defense Robert Gates said that intelligence showed Iran may have obtained the ability to launch dozens, even hundreds, of missiles targeting Europe.

The other conclusion was that the rapid development of U.S. missile defense capabilities and technologies would provide a more effective means for dealing with Iran's missile threat.

Based on the reevaluation, Obama decided to adjust the U.S. missile defense system, particularly the original plan for Eastern Europe. On September 17, 2009, he announced that the U.S. Government would abandon its plan of deploying a radar site in the Czech Republic and missile interceptors in Poland. Instead, it would implement a "phased adaptive approach" for European ballistic missile defense.

The new missile defense system the Obama administration is deploying is quite different from that of his predecessor. First of all, the scope of interception targets is dramatically expanded. The missile defense system Bush planned to deploy in Poland could only intercept intercontinental ballistic missiles. But the system that the Obama administration will eventually deploy there can protect the whole North Atlantic region from attacks of various distances and directions, including short- and medium-range missile attacks.

What's more, Obama will give priority to the use of sea-based interceptors. Standard Missile-3 (SM-3) interceptors became the first choice after one of the missiles, launched from a Navy cruiser in the Pacific, successfully shot down a failing U.S. satellite in space in February 2008.

The Pentagon's plan outlines, by 2011, the United States will deploy in Europe the sea-based Aegis combat system, SM-3 Block IA interceptors as well as a radar surveillance system. This is six to seven years earlier than the Bush administration's original plan.

Obama also highlights cooperation with other NATO member countries, while maintaining openness and strengthening anti-missile capabilities. By 2015, the United States will deploy more advanced SM-3 Block IB interceptors to respond to the threat of missiles within a range of 3,000 km.

And by 2018, even more advanced SM-3 Block IIA interceptors will be deployed to respond to the threat of missiles within a range of 5,500 km.

Further, by 2020, the United States will deploy SM-3 Block IIB interceptors to respond to the threat of various types of medium-range missiles and to the potential threat of intercontinental ballistic missiles. This should protect the whole of Europe.