Democratic contenders to be the next commander in chief named the Islamic State, the spread of nuclear weapons and climate change as some of the biggest threats facing the country in Tuesday night's debate.

Hillary Clinton said the threat from the spread of nuclear weapons is the most troubling. Martin O'Malley said Iran, the Islamic State and climate change are all significant problems facing the country. Bernie Sanders said climate change is the No. 1 threat for the U.S.

Lincoln Chafee, meanwhile, named "chaos in the Middle East" as the top threat. And Jim Webb said the U.S. relationship with China, cybersecurity and instability in the Middle East all threaten the U.S.

The answers come in contrast to top military leaders testifying before Congress this year, who mostly named Russia as the greatest military threat to America.

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