Answer:
Cuba had weapons aimed at the U.S.
For more details, look up the Cuban Missile Crisis.
One of the central issues in American history is the position of Abraham Lincoln with slavery. He expressed his opposition to slavery publicly and privately. He would not want to allow slavery to expand into any more territories as possible.
You could blame the global economy
Answer:
China, specifically Hong Kong. China is a communist nation that oppress their people in many ways. The Chinese are unable to freely express themselves, unable to have children as they please, and are under a strict code of religion. Like a volcano, the pressure has been building for years. One day, a revolution will free the oppressed Chinese and they will be free!