In this Sept. 8, 1945 file photo, an allied correspondent stands in the rubble in front of the shell of a building that once was a movie theater in Hiroshima, Japan, a month after the first atomic bomb ever used in warfare was dropped by the U.S. on Monday, Aug. 6, 1945. In a moment seven decades in the making, President Barack Obama this month will become the first sitting American president to visit Hiroshima, where the U.S. dropped an atomic bomb during World War II, decimating a city and exploding the world into the Atomic Age. (AP Photo/Stanley Troutman, File)

Seventy-five years ago, in August 1945, a radical, new era in warfare began with the first use of nuclear weapons.

A letter composed by Hungarian physicists and signed by Albert Einstein alerted President Franklin Roosevelt in August 1939 to the prospect of a powerful new weapon — an atomic bomb. The letter urged the U.S. to undertake research and development of such weapons, lest Nazi Germany accomplish this first. Einstein described the new technology during a subsequent meeting with the president at the White House.

The warnings were taken seriously and led to the Manhattan Project, a massive research and development program in collaboration with Canada and Great Britain. Secret, new facilities at multiple sites, including Oak Ridge, Tennessee, Hanover, Washington, and Los Alamos, New Mexico, played critical roles in the development of the first atomic weapons.

When Harry Truman became president on April 12, 1945, he had not been briefed on the atomic bomb. Roosevelt had met with his vice president only eight times since their election.

The first test of the new weapon on July 16 at a site near Alamogordo, New Mexico, generated a fireball and shock wave that stunned observers. Two days later, Truman, British Prime Minister Winston Churchill, and Chinese leader Chang Kai-shek issued the Potsdam Declaration, which called for Japan's immediate surrender and total disarmament. The document stated that Japan would not be destroyed. Its people would not be enslaved. Annihilation was the alternative. Japan did not respond.

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This July 16, 1945, file photo, shows the mushroom cloud of the first atomic explosion at Trinity Test Site near Alamagordo, N.M. A visit by The head of the National Nuclear Security Administration, Lisa Gordon-Hagerty coincides with the 75th anniversary of the Trinity Test in southern New Mexico, which marked the world's first atomic blast on July 16, 1945. She's scheduled to lead a commemoration Thursday, July 16, 2020, at the historic V-Site at Los Alamos National Laboratory, where early testing and some assembly of the atomic bomb took place. (AP Photo/File)

The decision to use the atomic bomb against Japan elicited strong and continuing controversy within the Truman administration. The final decision was influenced by high casualties sustained by U.S. forces in the fight to capture Okinawa. Military leaders estimated that U.S. forces would suffer a million casualties if Japan were invaded.

On Sept. 6 the first nuclear weapon was dropped on Hiroshima, a city of 350,000. Nicknamed "Little Boy," the bomb was equivalent to 15,000 tons of TNT. The bomb was delivered by a B-29 bomber, Enola Gay, piloted by Col. Paul Tibbets. An estimated 70,000 residents died instantly. A five-square mile area of the city was flattened.

Japan did not respond.

A second bomb, nicknamed "Fat Man," was dropped on Nagasaki by another B-29, Bocks Car, piloted by Maj. Charles Sweeney, on Sept. 9. The city was a secondary target, clouds having obscured the primary target, an armaments center at the city of Kokura. The 22-kiloton blast instantly killed 35,000 residents.

In addition to immediate fatalities, tens of thousands of residents of both cities died in subsequent months and years from radiation-related injuries and diseases.

John Hershey's "Hiroshima," published in 1946, is the best account of the human costs of a nuclear bombing.

The U.S. successfully tested an even more powerful hydrogen bomb in November 1952.

In August 1949, the Soviet Union conducted its first test of an atomic bomb, followed four years later by the detonation of a hydrogen bomb. The "cold war" took on dangerous, new dimensions.

In the early 1950s, schools regularly conducted "duck-and-cover" exercises. Students were instructed in how to protect ourselves in the event of a nuclear attack. Cities designated bomb shelters. Families built personal shelters.

The Treaty on the Non-proliferation of Nuclear Weapons, which was signed in August 1968 by the U.S., USSR, UK and 38 other nations, sought to limit the spread of nuclear weapons. The three major powers pledged that they would not assist other nations in development of such weapons. The treaty slowed but did not eliminate the spread of nuclear devices.

Today, the U.S., USSR, UK, France, China, Israel, India, Pakistan and North Korea possess nuclear weapons.

August 1945 introduced a persisting fear of unrestrained nuclear warfare that could threaten the very existence of mankind.

Contact Clif Cleaveland at