Three days later, on Aug. 9, another American bomber dropped a nuclear device on the city of Nagasaki, killing 40,000 to 80,000 people.

The devastation was followed by World War II's swift conclusion. It's seared into the collective global memory -- no other time in history has a nuclear weapon been used in war. The simple fact of the atomic bomb's awesome power went on to shape a half-century of Cold War geopolitics.

The justification for the Hiroshima and Nagasaki bombings remains the source of perennial historical study and debate. As the world marks the events' 70th anniversary this week, the legacy of what was first unleashed above Hiroshima now looms over newer conversations about disarmament and the nuclear programs of emerging powers.