In 1939, physicists Albert Einstein and Leo Szilard drafted a letter to US President Franklin D. Roosevelt, urging him to research atomic bombs before the Germans could build one first. By 1942, the United States had approved the top-secret Manhattan Project to build a nuclear reactor and assemble an atomic bomb.

Here, calutron operators work in Oak Ridge, Tennessee, in 1944. Calutrons were used to refine uranium ore into fissile material.

A billboard in Oak Ridge reminds workers of the top-secret nature of the facility.

Scientists with the Manhattan Project manually haul a container of radioactive material from a shed in Los Alamos, New Mexico.

Lie-detection tests were administered as part of security screening for the Manhattan Project.

These photos show some of the people who worked on the Manhattan Project at the Los Alamos National Laboratory in New Mexico. US Army Col. Leslie R. Groves, second row on the left, was appointed to head the project. Physicist J. Robert Oppenheimer led the lab at Los Alamos.

The United States detonated the world's first atomic bomb at a test site in New Mexico on July 16, 1945. Trinity was the code name of the test bomb, which was detonated in the Jornada del Muerto desert.

Groves and Oppenheimer examine twisted wreckage after the test blast. This is all that remained of a 100-foot tower, winch and shack that held the nuclear weapon.

Air Force Col. Paul Tibbetts waves from the pilot's seat of the Enola Gay moments before takeoff on August 6, 1945. A short time later, the plane's crew dropped the first atomic bomb in combat.

Eight days earlier, US President Harry Truman had warned Japan that the country would be destroyed if it did not surrender unconditionally.

A mushroom cloud billows over Hiroshima about one hour after detonation. At least 70,000 people were killed in the initial blast, while approximately 70,000 more died from radiation exposure. "The five-year death total may have reached or even exceeded 200,000, as cancer and other long-term effects took hold," according to the Department of Energy's history of the Manhattan Project.

A patient suffering severe radiation burns lies in the Hiroshima Red Cross hospital in August 1945. Many of those who survived the initial blast died of severe radiation-related injuries and illnesses.

This aerial photo shows the Japanese Army base at Hiroshima, a day after the bomb.

Bomb survivors lie in bed in Hiroshima.

A person's outline is seen on bank steps after the bombing in Hiroshima.

A man walks through the destruction in Hiroshima.

A worker stands next to an atomic bomb, code name "Fat Man," hours before it was dropped on Nagasaki, Japan, on August 9, 1945. The Hiroshima bomb was nicknamed "Little Boy."

This photo was taken about six miles from the scene of the Nagasaki explosion. According to the Nagasaki Atomic Bomb Museum, photographer Hiromichi Matsuda took this 15 minutes after the attack.

Bones and ashes are left behind after the bombing of Nagasaki, which killed up to 80,000 people.

A woman and a child walk in Nagasaki on the day of the bombing.

An aerial view of Nagasaki after the bombing.

A young survivor cries as he receives treatment at a temporary hospital in Nagasaki.

People in Osaka, Japan, listen to a radio broadcast of Emperor Hirohito on August 15, 1945. He announced that Japan had accepted the terms of the Potsdam Declaration, which called for the country's unconditional surrender.

US President Harry Truman announces Japan's surrender to media assembled at the White House.

A jubilant American sailor kisses a nurse in New York's Times Square as he celebrates the news that Japan has surrendered.

Soldiers and sailors on the USS Missouri watch as Japan's formal surrender is signed in Tokyo Bay on September 2, 1945.