President Barack Obama aspires to secure all nuclear weapons and materials within four years, an ambitious goal that deserves praise and won some converts at a historic two-day nuclear security summit this week in Washington.

China agreed to consider sanctions against Iran for that nation's reckless nuclear pursuits. Ukraine consented to dispose of its stockpile of highly enriched uranium. Chile has shipped its highly enriched uranium to the United States for safekeeping. And in a joint statement, attendees pledged to rein in nuclear materials by 2014.

However, it is one thing to preach to the choir and quite another to convert the sinner. As the president has noted, it is a "cruel irony" that the risk of a nuclear attack has grown since the end of the Cold War. The danger of terrorist organizations such as al-Qaida acquiring a nuclear bomb, or rogue nations joining the nuclear weapons club, is greater now than the 1960s risk of the United States exchanging nuclear missiles with the former Soviet Union.

The International Atomic Energy Agency has documented 18 cases of theft or loss of plutonium or highly enriched uranium, the crucial ingredients of nuclear weapons. Two months ago, for example, peace activists broke into a Belgian base where U.S. nuclear weapons are stored, and three years ago armed men unsuccessfully attacked a South African storage site that housed tons of highly enriched uranium.

Controlling nuclear materials isn't going to get any easier. Witness Venezuela, which under the erstwhile leadership of Hugo Chavez, covets a nuclear future.

Better global strategies for securing weapons and materials are needed, beginning with more nations demonstrating the will to relinquish nuclear ambitions, weapons and materials. Although at least 17 nations, including Libya, Turkey and Taiwan, have eliminated weapons-usable nuclear material since 1992, at least 38 nations have not. With so much at stake, it is disappointing that Syria wasn't invited to Washington, and that Israel, which is believed to possess nuclear weapons, sent lower-level diplomats to avoid a high-level confrontation on its nuclear status.

More diplomatic heavy lifting will be required next month at the Nuclear Nonproliferation Treaty Review Conference in New York, and later this year when G-8 and G-20 nations meet in Canada. Closer to home, Congress also needs to support the administration's proposal to increase domestic spending for global nuclear security to roughly $2 billion in fiscal 2011.

Nuclear security will not be achieved unless there is an overwhelming will among nations to devote resources toward securing and reducing stockpiles. The Washington summit and coming global meetings are watershed opportunities that mustn't be squandered. The threat is real, and not taking the steps now to diminish it could be cataclysmic.