War

Nuclear war
The largest threat to the wealthier nations, are currently the crazed dictator of North Korea who has made videos threatening to nuke America, now has nuclear weapons, and is working on making increasingly long range missiles to carry them. He doesn't mind letting his own people starve to death while he spends money to live in luxury and fund his costly weapons program. He may sell nuclear secrets to other nations or groups, and get them to attack America for him instead.

Radiation pills can treat you from radiation poisoning from the nuclear fallout. A nuclear winter wouldn't freeze the entire world over, but could cool off the world enough to cause crop failure in many areas leading to starvation around the world.

A single nuclear warhead set off in the atmosphere could cause an electromagnetic pulse to devastate America.

Genetically engineered virus
China is researching genetic engineering to a great extent. If they made a virus that only killed people of certain races, everyone that wasn't Asian, then they could wipe out much of the world's population while not affecting themselves at all. A nation that allows millions of its people to die from pollution every year, can not really be trusted to be reasonable, civilized, and stable.

Radar invisible air attacks
America has admitted that one of its RQ-170 Sentinel radar invisible spy drones was captured by Iran in 2011 while flying along the Afghanistan and Iranian border. Some claimed they somehow sent signals that took control of it. Doesn't matter how they got it though. If they used an electron microscope to figure out the molecular structure of the radar invisible material, how long would it take for them to duplicate it? Would they share it with other nations? Have other nations captured this technology as well?

Could terrorists use radar invisible aircraft to fly over America at night, and spread bombs at various targets, firebombs to burn cities to the ground starting more fires at once than anyone could put out, or release viruses upon us? What about dirty bombs, or the nations that are researching nuclear bombs and may already have them?

Update: China has radar invisible drones, the missing drone said to have been sold to them to reverse engineer. No telling who else would be sold this technology or gain it by other means.