Invaders from the outside. Humans instinctively live in bands and group bands together to compete with other groups of bands. This means local wars are a part of human nature. Local wars tend to grow to regional and regional tend to grow to world conflicts.
So WW-3 was called the Cold War that was not cold depending on where you lived. And now we are in the beginning stages of WW-4 as Jihadists enter their historical crusade phase.
In the 80's Reagan and Thatcher ran the world. When the U.S. polices the world and has a strong President, you don't have cockroaches like ISIS, the Taliban, Al Quada, Boka Haram and all the other terrorist gangs running around. This worldwide unrest only happens when you have a weak, America hating, Muslim supporter in the office. Obama has done all this on purpose, so WWIII can break out. He's despicable!