Harry Truman

Sixteen hours ago an American airplane dropped one bomb on Hiroshima and destroyed its usefulness to the enemy. . . . 

It was to spare the Japanese people from utter destruction that the ultimatum of July 26 was issued at Potsdam.  Their leaders promptly rejected that ultimatum.  If they do not now accept our terms they may expect a rain of ruin from the air, the like of which has never been seen on this earth.

From the White House press release announcing the bombing of Hiroshima on this date in 1945.

On July 26, 1945, the leaders of the US, UK, and nationalist China issued the Potsdam Declaration, a statement that demanded the unconditional surrender of all Japanese armed forces.  The ultimatum concluded with these words:  "The alternative for Japan is prompt and utter destruction."  

The "Little Boy" atomic bomb that was dropped on Hiroshima killed about 70,000 people (30% of the city's population) immediately.  As many as 200,000 people may have eventually died from burns, radiation, cancer, and other bomb-related causes.  The bombing of Nagasaki three days later resulted in a somewhat smaller number of deaths.

The Soviet Union declared war on Japan and invaded Manchuria with an army of about 1.7 million men several hours before the Nagasaki bomb was dropped.  The Soviet invasion may have been the more immediate cause of Japan's surrender -- until the Soviets declared war, Japan had hoped the Soviet Union might help it negotiate acceptable peace terms with the Allies.