Weapon of mass destruction

A weapon of mass destruction (WMD) is a biological, chemical, radiological, nuclear, or any other weapon that can kill or significantly harm many people or cause great damage to artificial structures (e.g., buildings), natural structures (e.g., mountains), or the biosphere. The scope and usage of the term has evolved and been disputed, often signifying more politically than technically. Originally coined in reference to aerial bombing with conventional explosives during World War II, it has later come to refer to large-scale weaponry of warfare-related technologies, primarily biological, chemical, radiological, or nuclear warfare. Protective measures against weapons of mass destruction are known as CBRN defense.

The histories of biological and chemical warfare date from antiquity to the modern period, with toxic gases used on a vast scale in World War I. In World War II, the United States first developed nuclear weapons, and used them in war twice, in the atomic bombings of Hiroshima and Nagasaki. Nazi Germany's use of gases during the Holocaust killed approximately three million people, the largest death toll due to chemical weapons in history. The Empire of Japan used chemical and biological warfare on a large scale in China during the Second Sino-Japanese War, killing hundreds of thousands.

During the Cold War, the United States and Soviet Union operated the largest programs for nuclear, chemical, and biological weapons in history. The nuclear arms race saw the production of tens of thousands of thermonuclear weapons and delivery systems such as intercontinental ballistic missiles. Their chemical weapons programs focused on sarin, VX/VR, and mustard gas.