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. Biotechnology was used by both countries to enhance pathogens, such as lethal agents that cause anthrax and incapacitating agents that cause glanders. Both countries briefly tested radiological weapons, but no state is known to have mass-produced or used them. The US Nunn–Lugar Cooperative Threat Reduction program was largely successful in securing ex-Soviet weapons of mass destruction infrastructure and personnel following the dissolution of the Soviet Union, and denuclearizing Belarus, Ukraine, and Kazakhstan.

The 1995 Tokyo subway sarin attack, September 11 attacks, and 2001 anthrax attacks brought a heightened concern of weapons of mass destruction and terrorism. Ba'athist Iraq had carried out mustard and nerve agent attacks in the Iran–Iraq War, killing thousands of civilians and troops. Following the 2002 Iraq disarmament crisis, false claims by a US-led coalition that Iraq was maintaining its weapons of mass destruction programs played a major role in justifying the 2003 invasion of Iraq. Ba'athist Syria used chemical weapons during the Syrian civil war, killing thousands and prompting US-led airstrikes in 2017 and 2018.

As of 2025, nine countries possess nuclear weapons, while 185 countries pledge not to acquire them via the 1968 Non-Proliferation Treaty. The 1925 Geneva Protocol outlawed the use, but not stockpiling, of chemical and biological weapons. The 1972 Biological Weapons Convention and 1993 Chemical Weapons Convention seek complete elimination by all parties, but are challenged by violations and by a small number of non-signatory states such as Egypt, Israel, and North Korea.