For many people now living, the modern world began on August 6, 1945. The U.S. atomic bombings of the Japanese cities of Hiroshima and, three days later, Nagasaki, brought an end to World War II. But the arrival of the Atomic Age, and the brutal evidence of just how effective this new weapon was, tainted the ensuing peace. Edward R. Murrow, a famed U.S. radio journalist of the time, commented: "Seldom if ever has a war ended leaving the victors with such a sense of uncertainty and fear, with such a realization that the future is obscure and that survival is not assured."

A brief bomb history

Scientists knew about the atom's basic structure as early as the late 1800s. But only six years elapsed between the discovery of fission in 1939 and the destruction of Hiroshima by an atomic bomb.

By the 1930s, physicists were aware of the potential military uses of nuclear energy. In 1939, German-born scientist Albert Einstein sent a letter to U.S. President Franklin Roosevelt alerting him to the possible threat. Soon after the U.S. entered World War II, the United States government secretly established the Manhattan Project to develop an atomic bomb.

The scientists assembled to work on the atomic weapon began to succeed. In late 1942, Italian Nobel laureate Enrico Fermi and his associates demonstrated the first self-sustaining chain reaction in a laboratory built under a squash court at the University of Chicago. The Manhattan Project then set up facilities for nuclear production and research -- which quickly saw results. The first true plutonium production reactor began operation in Oak Ridge, Tennessee, in November 1943.

On July 16, 1945, Manhattan Project scientists detonated the world's first nuclear device at the Trinity test site near Alamogordo, New Mexico. Three weeks later, "Little Boy" was dropped on Hiroshima.

How the bomb works

Nuclear weapons are the most powerful explosive devices yet devised by man. The are broken down into two categories -- fission, or "atomic" bombs, and fusion, the so-called "hydrogen" or "thermonuclear" weapons.

An atomic bomb gets its destructive power from a sudden release of energy created by nuclear fission. In fission weapons, matter is changed into energy when the nucleus, or core, of an atom is split apart. As the nucleus breaks apart, it releases sub-atomic neutrons, which in turn collide with and break up other atomic nuclei. This is the chain reaction, the event that gives nuclear weapons a power far greater than conventional weapons.

The first atomic bombs used two basic methods to create a nuclear explosion.

"Little Boy," the bomb that destroyed Hiroshima, used a so-called gun-barrel method. A small explosion drove one piece of uranium into another with ferocious impact in a device similar to an artillery barrel -- creating the chain reaction.

"Fat Man," the bomb dropped on Nagasaki, was an implosion device -- a mass of plutonium-239 surrounded by a chemical explosive. The weapon's fuse made sure all the conventional, chemical explosive was detonated at the same time. That explosion compressed the plutonium, setting off the chain reaction.