World War III
A world war means a war that is fought by multiple countries of the world against each other, across different continents. Because technology and weapons have become so advanced, most people agree that if this war ever happens, nuclear, biological and chemical weapons will be used. Nuclear weapons get their energy from breaking or joining atoms. Biological weapons are living things, usually bacteria or viruses. Chemical weapons might not kill quickly, but poison people or their land. Together, nuclear, biological and chemical weapons are called Weapons of mass destruction. Conventional weapons on the other hand are "normal" ones, like guns or bombs.
Mass destruction could damage much of the Earth and kill most humans and many other living things. Albert Einstein said, "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." This shows that he believed that civilization as we know it would be gone.