"Some argue that the spread of these weapons cannot be checked - that we are destined to live in a world where more nations and more people possess the ultimate tools of destruction," Obama said to a bustling crowd of more than 20,000 in an old square outside the Prague Castle gates.
"This fatalism is a deadly adversary," he said. "For if we believe that the spread of nuclear weapons is inevitable, then we are admitting to ourselves that the use of nuclear weapons is inevitable."
Obama targeted his comments at one point directly at North Korea, which launched a rocket late Saturday night in defiance of the international community. The president was awoken by an aide and told of the news, which occurred in the early morning hours in Prague.
"North Korea broke the rules once more by testing a rocket that could be used for a long range missile," Obama said. "This provocation underscores the need for action - not just this afternoon at the UN Security Council, but in our determination to prevent the spread of these weapons."
At a summit with leaders of the European Union later in the day, Obama called for a swift, joint statement condemning North Korea's actions, and said the foreign ministers from the countries were in the process of crafting one.
Addressing another potential nuclear foe, Obama said in his speech the U.S. will present Iran with "a clear choice" to join the community of nations by ceasing its nuclear and ballistic missile activity or face increased isolation and a nuclear arms race in the Middle East.
He said the U.S. will proceed with development of a missile defense system in Europe as long as there is an Iranian threat of developing nuclear weapons. If that threat is removed, he said, "The driving force for missile defense in Europe will be removed."
The choice of Prague for such a speech carried large symbolism, and Obama didn't ignore it. Decades of communism were toppled in Czechoslovakia through the 1989 Velvet Revolution, so named because it was one of the few peaceful overthrows of communism in the Iron Curtain. The Czech Republic split from Slovakia in 1993.
Obama praised the Czechs for helping "bring down a nuclear-armed empire without firing a shot."
Obama coupled his call for a nuclear-free world with an assurance that America would not unilaterally give up nuclear weapons. It must be a one-for-all, all-for-one endeavor, he said, and until that is possible, the U.S. will maintain a big enough arsenal to serve as a deterrent.
Few experts think it's possible to completely eradicate nuclear weapons, and many say it wouldn't be a good idea even if it could be done. But a program to drastically cut the world atomic arsenal carries support from scientists and lions of the foreign policy world.
The Comprehensive Test Ban Treaty was signed by former President Bill Clinton but rejected by the Senate in 1999. Over 140 nations have ratified the ban, but 44 states that possess nuclear technology need to both sign and ratify it before it can take effect and only 35 have do so. The United States is among the key holdouts, along with China, Egypt, India, Indonesia, Iran, Israel, North Korea, and Pakistan.
Ratification of the test ban was one of several "concrete steps" Obama outlined as necessary to move toward a nuclear-free world, He also called for reducing the role of nuclear weapons in American national security strategy, negotiating a new strategic arms reduction treaty with Russia, and seeking a new treaty to end the production of fissile materials used in nuclear weapons.
Obama also said the U.S. will seek to strengthen the nuclear nonproliferation treaty by providing more resources and authority for international inspections and mandating "real and immediate consequences" for countries that violate the treaty.
Obama spoke after conferring with Czech leaders. He is nearing the end of a sweep through five nations in Europe, pivoting from the global economic swoon to the war in Afghanistan to, now, the crisis in North Korea and the fate of the nuclear world.
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