WASHINGTON -- Leaders from 47 nations are meeting in Washington, D.C., Monday for a two-day summit on nuclear security.

While summit leaders are hoping to stop nuclear terrorism, groups like al Qaeda have other ideas.

Heading Off Armageddon

With al Qaeda having made no secret of its desire to acquire nuclear weapons and U.S. intelligence officials having long warned about that doomsday scenario, world leaders are now taking notice.

Monday's summit is the largest assembly of foreign leaders gathered by an American president since 1945.

"The single biggest threat to U.S. security, both short-term, medium term and long term would be the possibility of a terrorist organization obtaining a nuclear weapon," President Obama said. 

The president's goal is to secure all loose nuclear materials within four years, in order to prevent them from falling into the hands of terrorists.

"We often say that the threat of nuclear war, as we used to think about it during the Cold War, has actually decreased. But the threat of nuclear terrorism has increased," Secretary of State Hillary Clinton said.

The president's larger aim is a world free of nuclear weapons, and he is taking the lead on that initiative.

Obams slashed America's own nuclear arsenal last week and announced that the U.S. will cease developing nuclear weapons. Under this new strategy, America would not use nukes against countries that do not have them, even in response to a biological or chemical attack on U.S. soil.

Iran, North Korea Scoff

However, Iran and North Korea were not impressed with the president's moves to disarm. Both countries slammed him as an aggressor and have shown no signs of slowing down their own nuclear weapons programs.

During the summit, Obama will attempt to convince leaders from Russia and China to support sanctions against Iran.

Meanwhile, Israeli Prime Minister Benjamin Netanyahu decided to skip the summit when it became clear that Muslim nations led by Turkey would use the meeting to slam Israel over its nuclear program.