Another card in the tower to watch is Israel. If the U.S. ally thinks sanctions are failing, it could turn to military action to delay Iran's nuclear intentions.
Al-Qaeda/War on Terror. "The one thing we've seen about Al-Qaeda affiliates is that they don't give up on a tactic," Carafano says.
After a string of bombings, the most recent being the assault on the American consulate in Benghazi, the specter of another attack by the terrorist group remains ever present.
"You've got to ask, 'Are they going to be looking for another embassy?'" Carafano says.
China. Tension over trade agreements and alleged currency manipulation by China has soured the United States' relationship with the second-largest economy in the world. Add on China's growing military power and increasing political influence, and dealing with the Asian giant will be high on the priority list for the next president.
To add to the complexity, China's economic landscape is changing. China remains a manufacturing mecca for multinational companies, but workers there are no longer willing to work in subpar conditions for little pay. Higher wages in China have helped move many factory operations into Indonesia, India, and Bangladesh.
That's meant a slowdown for China's economy, which has far-reaching impacts for other economies around the world, including the United States.
"Any moves from this giant creates a huge wake that will quickly wash onto American shores," a recent NBC article noted.
Meg Handley is a reporter for U.S. News & World Report. You can follow her on Twitter or reach her at firstname.lastname@example.org.