LINDA WERTHEIMER, host: It's ALL THINGS CONSIDERED from NPR News. Guy Raz is away. I'm Linda Wertheimer.</s>LINDA WERTHEIMER, host: The Nuclear Security Summit begins in Washington tomorrow. Forty-seven countries are participating, most of them represented by heads of state or government. This is an important moment in President Obama's effort to keep his campaign pledge to work toward a world free from the threat of nuclear weapons.</s>President BARACK OBAMA: It's time to send a clear message to the world: America seeks a world with no nuclear weapons. As long as nuclear weapons exist, we'll retain a strong deterrent, but we will make the goal of eliminating all nuclear weapons a central element in our nuclear policy.</s>LINDA WERTHEIMER, host: That was candidate Obama at Purdue University in the summer before his election. In the meeting that he will convene tomorrow in Washington, the president will ask nations to secure all nuclear materials within four years, anything that might be used by terrorists or even nations to make a weapon.</s>LINDA WERTHEIMER, host: Tomorrow, Russia's president is expected to sign an agreement on how to dispose of Cold War-era nuclear weapons. The U.S. and Russia have already signed the START treaty, pledging to cut each country's nuclear arsenals, but some members of Congress are expressing doubts about the president's plans.</s>Senator JOE LIEBERMAN (Democrat, Connecticut): I'm going to be real hesitant to vote for this treaty unless we have a commitment from the administration that they are prepared to modernize our nuclear stockpiles.</s>LINDA WERTHEIMER, host: That was Senator Joe Lieberman of Connecticut. But the president has made the point several times that the threat from nuclear weapons is changing, and this week's summit is meant to address that.
Both nuclear proliferation and disarmament are in the headlines. This past week, the American and Russian presidents signed the newest START treaty, pledging to shrink each country's nuclear arsenal. And now, heads of state from almost 50 countries are arriving in Washington for a landmark Nuclear Security Summit. Host Linda Wertheimer reviews the developments in world of nuclear security.