When I was born, the world was divided, and our nations were faced with very different circumstances. Few people would have predicted that someone like me would one day become an American President. Few people would have predicted that an American President would one day be permitted to speak to an audience like this in Prague. And few would have imagined that the Czech Republic would become a free nation, a member of NATO, and a leader of a united Europe. Those ideas would have been dismissed as dreams.

We are here today because enough people ignored the voices who told them that the world could not change.

We are here today because of the courage of those who stood up - and took risks - to say that freedom is a right for all people, no matter what side of a wall they live on, and no matter what they look like.

We are here today because of the Prague Spring - because the simple and principled pursuit of liberty and opportunity shamed those who relied on the power of tanks and arms to put down the will of the people.

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Just as we stood for freedom in the 20th century, we must stand together for the right of people everywhere to live free from fear in the 21st. And as a nuclear power - as the only nuclear power to have used a nuclear weapon - the United States has a moral responsibility to act. We cannot succeed in this endeavor alone, but we can lead it.

So today, I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons. This goal will not be reached quickly - perhaps not in my lifetime. It will take patience and persistence. But now we, too, must ignore the voices who tell us that the world cannot change.

First, the United States will take concrete steps toward a world without nuclear weapons.

To put an end to Cold War thinking, we will reduce the role of nuclear weapons in our national security strategy and urge others to do the same. Make no mistake: as long as these weapons exist, we will maintain a safe, secure and effective arsenal to deter any adversary, and guarantee that defense to our allies - including the Czech Republic. But we will begin the work of reducing our arsenal.

To reduce our warheads and stockpiles, we will negotiate a new strategic arms reduction treaty with Russia this year. President Medvedev and I began this process in London, and will seek a new agreement by the end of this year that is legally binding, and sufficiently bold. This will set the stage for further cuts, and we will seek to include all nuclear weapons states in this endeavor.

To achieve a global ban on nuclear testing, my Administration will immediately and aggressively pursue U.S. ratification of the Comprehensive Test Ban Treaty. After more than five decades of talks, it is time for the testing of nuclear weapons to finally be banned.

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I know that there are some who will question whether we can act on such a broad agenda. There are those who doubt whether true international cooperation is possible, given the inevitable differences among nations. And there are those who hear talk of a world without nuclear weapons and doubt whether it is worth setting a goal that seems impossible to achieve.

But make no mistake: we know where that road leads. When nations and peoples allow themselves to be defined by their differences, the gulf between them widens. When we fail to pursue peace, then it stays forever beyond our grasp. To denounce or shrug off a call for cooperation is an easy and cowardly thing. That is how wars begin. That is where human progress ends.

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