Five Disasters That Could Surprise America

U.S. President Barack Obama begins his second term at a critical moment in world affairs -- al Qaeda raising its head in North Africa, President Bashar al-Assad possibly preparing to use chemical weapons in Syria, Iran moving toward the nuclear weapons threshold, and tensions rising in Asia. An unstable world promises to present the president with many challenges in the next four years, and his advisors are already grappling with how to confront them. Some looming challenges -- like the America's debt or China's rise -- have been the focus of a good deal of attention. However, low-probability but high-impact "black-swan" events could also define Obama's second term, diverting the president from his intended foreign-policy agenda. These events would be so catastrophic that he needs to take steps now to minimize the risk that they might occur. Here are some of the black swans that could upend the Obama administration's agenda over the next four years: