President Barack Obama last night launched a drive to rid the world of nuclear weapons.
He said the United States, as the only country to have used nuclear bombs in warfare, had a moral responsibility to lead the campaign to abolish them.
Nuclear arms reduction will become a strong element of his foreign policy, particularly in terms of nuclear terrorism, and he is using his visit to Europe to further that aim.
"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," President Obama told a huge crowd in a square outside the Prague Castle gates.
"This fatalism is a deadly adversary. If we believe the spread of nuclear weapons is inevitable, then we are admitting to ourselves that the use of nuclear weapons is inevitable."
His new direction is also likely to increase his credibility in dealing with the ambitions of countries such as Iran and North Korea, which yesterday launched a long-range rocket the West says could carry missiles.
He said last night in Prague that the launch underscored the urgency of his weapons policy.
North Korea broke the rules and "now is the time for a strong international response" .
President Obama said he would convene an international summit this year to look at the eventual elimination of nuclear weapons.
During the G20 summit in London last week, he and Russian President Dmitry Medvedev agreed to discuss this year making further cuts in their own arsenals.
Prime Minister John Key last night welcomed the move as a "step in the right direction."
"It is an ambitious goal and it is not to be under-estimated how difficult it would be to achieve but nevertheless a positive move."
Labour leader Phil Goff, a former Disarmament Minister and Foreign Minister, described the shift in more heroic terms.
"The vision Obama is signalling is some basis for hope that again the world can move to rid the world of weapons of mass destruction that have the potential to cause massive loss of human life and even potentially the elimination of human life," Mr Goff said.
The new US direction could extend areas of potential co-operation with New Zealand and is likely to be discussed on Wednesday when Foreign Minister Murray McCully meets US Secretary of State Hillary Clinton in Washington.
Mr Obama is pledging a greater commitment to the 1968 Non-Proliferation Treaty under which the US, Britain, France, Russia and China would progressively reduce their stockpile of weapons in exchange for other countries not developing nuclear arms.
India and Pakistan have not signed the treaty and have nuclear weapons.
Israel has not signed, and will not say whether it has nuclear weapons.