Before the Second World War, military conflicts were fought under orthodox conditions, usually termed "conventional warfare," but several innovations had significantly changed combat, leading inextricably to the race for a nuclear weapon in the 1930s and 1940s. Conflicts had been fought by armies on horseback with guns of varying sophistication since the 16th century, but mechanized warfare and machine guns changed this calculus and set the stage for future combat by the end of World War I. Other sinister changes entered the fray during this conflict, such as chemical weapons like chlorine and mustard gas. The total warfare brought about by World War I and ensuing wars like the Spanish Civil War made the quest for the most powerful weapons somewhat necessary.

The Manhattan Project would ultimately yield the "Little Boy" and "Fat Man" bombs that released more than 100 Terajoules of energy at Hiroshima and Nagasaki, but as it turned out, the Axis were not far behind with their own nuclear weapons program. When the Nazis' quest for a nuclear weapon began in earnest in 1939, no one really had a handle on how important nuclear weapons would prove to war and geopolitics. The attacks on Hiroshima and Nagasaki in August 1945, along with the Cold War-era tests and their accompanying mushroom clouds, would demonstrate the true power and terror of nuclear weapons, but in the late 1930s these bombs were only vaguely being thought through, particularly after the successful first experiment to split the atom by a German scientist. The nuclear age itself was in its infancy, barely 35 years old, but within a few short years the advent of nuclear war loomed over the world and the prospect of the enemy winning the nuclear race kept Allied leaders awake at night.