PRAGUE -- Declaring the future of mankind at stake, President Obama on Sunday said all nations must strive to rid the world of nuclear arms and that the U.S. had a "moral responsibility" to lead because no other country has used one.

A North Korean rocket launch upstaged Obama's idealistic call to action, delivered in the capital of the Czech Republic, a former satellite of the Soviet Union. But Obama dismissed those who say the spread of nuclear weapons, "the most dangerous legacy of the Cold War," cannot be checked.

"This goal will not be reached quickly -- perhaps not in my lifetime," he told a cheering crowd of more than 20,000 in the historic square outside the Prague Castle gates. We "must ignore the voices who tell us that the world cannot change. We have to insist, 'Yes, we can.'"

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. Even backward nations such as North Korea have shown they can develop bombs, given enough time.

But a program to drastically cut the world atomic arsenal carries support from scientists and lions of the foreign policy world. Obama embraced that step as his first goal and chose as the venue for his address a nation that peacefully threw off communism and helped topple the Soviet Union, despite its nuclear power.

But he said his own country, with its huge arsenal and its history using two atomic bombs against Japan in 1945, had to lead the world. He said the U.S. has a "moral responsibility" to start taking steps now.

"To reduce our warheads and stockpiles, we will negotiate a new Strategic Arms Reduction Treaty with the Russians this year," he promised.

The nuclear-free cause is more potent in Europe than in the United States, where even Democratic politicians such as Obama must avoid being labeled as soft or naive if they endorse it. Still, Obama said he would resubmit a proposed Comprehensive Test Ban Treaty to the Senate for ratification. The pact was signed by President Bill Clinton but rejected by the Senate in 1999.

While espousing long-term goals, Obama took care to promise that America would not lower its defenses while others are pursuing a nuclear threat. He warned both North Korea, which has tested a nuclear weapon, and Iran, which the West says is developing one, that the world was against them.

Obama gave his most unequivocal pledge yet to proceed with building a missile defense system in Europe, so long as Iran pursues nuclear weapons, a charge it denies. That shield is to be based in the Czech Republic and Poland. Those countries are on Russia's doorstep, and the missile shield has contributed to a significant decline in U.S.-Russia relations.

In the interest of resetting ties with Moscow, Obama previously had appeared to soft-pedal his support for the Bush-era shield proposal. But he adopted a different tone in Prague.

"As long as the threat from Iran persists, we will go forward with a missile defense system that is cost-effective and proven," Obama said, earning cheers from the crowd.

Hours before the address, an aide awoke Obama in his hotel room to tell him that North Korea had make good on its pledge to launch a long-range rocket. By lunchtime, the president had addressed it publicly nearly half a dozen times.

"Rules must be binding," he said. "Violations must be punished. Words must mean something."

"Now is the time for a strong international response," he said.

After the speech and a round of private meetings with foreign leaders, Obama arrived in Turkey, the final stop of his trip.

On the broader anti-nuclear issue, more than 140 nations have ratified the Comprehensive Test Ban Treaty. 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 holdouts, along with China, Egypt, India, Indonesia, Iran, Israel, North Korea, and Pakistan.

Ratification 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 and seeking a new treaty to end the production of fissile materials used in nuclear weapons.

Obama 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.

He offered few details of how he would accomplish his larger goal and acknowledged that "in a strange turn of history, the threat of global nuclear war has gone down, but the risk of a nuclear attack has gone up."