Nuclear warfare (sometimes atomic warfare or thermonuclear warfare) is a military conflict or political strategy in which nuclear weaponry is used to inflict damage on the enemy. Compared to conventional warfare, nuclear warfare can be vastly more destructive in range and extent of damage, and in a much shorter time. A major nuclear exchange would have long-term effects, primarily from the fallout released, and could also lead to a "nuclear winter" that could last for decades, centuries, or even millennia after the initial attack. Some analysts claim that with this potential nuclear winter side-effect of a nuclear war almost every human on Earth could starve to death. Other analysts, who dismiss the nuclear winter hypothesis, calculate that with nuclear weapon stockpiles at Cold War highs, in a surprise countervalue global nuclear war, billions of casualties would have resulted but billions of people would nevertheless have survived.

So far two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6, 1945, a uranium gun-type device (code name "Little Boy") was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium implosion-type device (code name "Fat Man") was detonated over the Japanese city of Nagasaki. These two bombings resulted in the deaths of approximately 129,000 civilians and military personnel.