Atomic explosions rely on the same principle that generates nuclear energy, which is that the potential energy of matter is equal to its mass multiplied by the square of the speed of light. In the case of most nuclear weapons, this involves the splitting of a uranium atom, which unleashes a massive explosion of energy.
The energy of an atomic bomb is released in the form of heat, and the sudden expansion of that wave of heat ignites the air around it, which results in a colossal fireball and the release of toxic levels of electromagnetic radiation. The original construction of the first atomic bomb began when several European scientists alerted the United States government that Nazi Germany was in the process of refining radioactive materials for use in a nuclear detonation. This began one of the most potentially destructive arms races in human history, which culminated in the destruction of two major Japanese cities.
Years later, the memory of the havoc wrought on Nagasaki and Hiroshima is often what stays the hand of world leaders when the use of strategic nuclear arsenals becomes an option. To date, no other nuclear strike has been made by any other country, and while many nuclear arsenals have been reduced, the threat of the use of nuclear weapons remains.