6 THINGS TO KNOW ABOUT THE ATOMIC BOMB.

• Author: Elder, Robert K.
• Position: TIMES PAST: 1945

Seventy-five years ago, the U.S. dropped two nuclear bombs on Japan, ushering in the atomic age. The world hasn't been the same since.

The giant purple mushroom cloud boiled 45,000 feet high over the Japanese city of Hiroshima. The death and destruction left in its wake were unlike anything the world had ever seen: 90 percent of the city wiped out in the blink of an eye.

The U.S. dropped the world's first nuclear bomb on Hiroshima on August 6,1945, and then, three days later, a second bomb on Nagasaki. Those bombings 75 years ago forced Japan to surrender, bringing an end to World War II (1939-45).

But the cost was massive. The combined death toll was estimated to be between 129,000 and 226,000 Japanese citizens--mostly civilians. The bombings also changed warfare and national security forever. They ushered in the atomic age, which has made the world a more complex and dangerous place, as nuclear weapons have spread around the globe.

From schoolchildren crouching under their desks in radioactive fallout drills during the Cold War to nuclear threats from North Korea today, the world hasn't been the same since.

Here are six important things to know about the atomic bombings that changed history.

1 The U.S. nuclear program all started with a letter from Albert Einstein.

In 1939, when Germany's dictator, Adolf Hitler, invaded Poland, he set into motion the largest, deadliest global conflict in history: World War II.

That year, Albert Einstein--the famous, Nobel Prize-winning Jewish physicist who had escaped Nazi Germany--and the Hungarian-born physicist Leo Szilard wrote a letter to President Franklin D. Roosevelt. They warned that Germany had embarked on a program to develop a nuclear device, which could harness the destructive energy released during fission--the splitting of an atom.

"A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory," the scientists wrote.

That letter, and two follow-ups, would prompt President Roosevelt to authorize study into fission reactions. By 1942, the U.S. had entered the war after Japan's surprise attack on Pearl Harbor.

And the race to unlock the secrets of the atom would heat up, evolving into the Manhattan Project. The top-secret mission to develop a nuclear, or atomic, bomb, led by the U.S., involved more than 130,000 people in 30 locations across North America and Britain.

What both Einstein and Szilard underestimated, however, was the destructive power of a nuclear bomb. Einstein, a lifelong pacifist, would come to regret his letters to Roosevelt.

"Had I known that...