THE MANHATTAN PROJECT: As North Korea and the U.S. edge closer to a nuclear conflict, a look at how the atomic age began 75 years ago.

• Author: Brown, Bryan
• Position: TIMES PAST 1942

North Korea's dictator, Kim Jong Un, vows to annihilate the U.S. with a "hail of fire." President Trump threatens to "totally destroy North Korea." The tensions have brought the world closer to a nuclear conflict than at any time since I the end of the Cold War (1947-91).

Although North Korea's ability to attack the U.S. mainland I with nuclear missiles is still an open question, any war waged I with atomic weapons could have horrific consequences, I leveling whole cities and killing millions of people.

Where did such monstrous firepower come from? It all began 75 years ago, in September 1942, when the U.S. Army took command of a top-secret effort called the Manhattan Project. The project's goal was to produce a powerful new weapon that could be used against America's enemies in World War II (1939-45). The result would be the world's first atomic bomb. In 1945, U.S. warplanes dropped two atom bombs, on the Japanese cities of Hiroshima and Nagasaki, bringing Japan to its knees and quickly ending the war--and changing the nature of warfare forever.

Today, as nuclear fears again make headlines, we're still grappling with the destructive power that the Manhattan Project unleashed.

The Nazi Threat

World War II began in Europe when German dictator Adolf Hitler attacked neighboring Poland in September 1939, with a goal of conquering all of Europe. Only a month before, German-born physicist Albert Einstein--a Jew who fled the Nazis for the U.S.--had written to President Franklin D. Roosevelt, warning that German scientists might be using new advances in physics to try to engineer a nuclear weapon. Einstein explained that building such a weapon would involve splitting the nucleus of an atom, a process called fission. Using the element uranium, this process could cause a chain reaction that would unleash an amount of energy millions of times more destructive than dynamite.

Roosevelt ordered a study. Then in December 1941, Japan attacked the U.S. Naval base at Pearl Harbor, Hawaii. The U.S. joined World War II against Japan and its ally Germany, and Americans were soon fighting and dying in Europe and Asia. The quest for an atomic bomb took on a new urgency.

About 500,000 people, including thousands of scientists, were tapped for the effort, named for the location of its first office, in New York City. Secret labs and test sites were scattered in places including Tennessee, Washington State, and New Mexico.

"The Manhattan Project was so massive that its tendrils stretched throughout the military and scientific communities," says Karen Harpp, a nuclear weapons expert at Colgate University in Hamilton, New York.

The technical challenges of making such a bomb were immense. "Because they were doing this for the first time, it wasn't even clear that it could be made to work correctly," says Alex Wellerstein, a nuclear historian at Stevens Institute of Technology in Hoboken, New Jersey. And then, he says, "It had to be made small enough to fit inside of an airplane."

Finally, in July 1945, at a remote spot in New Mexico called Alamogordo, the first atomic bomb was detonated. The explosion, which had the force of 20,000 tons of dynamite, generated a giant mushroom cloud crowned by a ball of fire that reached 30,000 feet into the air.

"We knew the world would never be the same," lead scientist J. Robert Oppenheimer later said.

Officials were relieved. The bomb worked. Now, what would they do with it? That grave decision fell to the new president, Harry S. Truman. (Roosevelt had died in office in April, never having told his vice president about the Manhattan Project. Truman learned of it only after becoming president.)

The Germans had already surrendered in May, ending the war in Europe. But the Japanese kept fighting. Truman had to decide whether to risk an invasion of Japan that could claim hundreds of thousands of American lives--or drop the bomb.

Truman would later say it was an easy decision to use the new weapon. On August 6, a U.S. warplane dropped a bomb nicknamed "Little Boy" on Hiroshima. The explosion destroyed more than five square miles of the city. Some 80,000 people were killed instantly. Tens of thousands more would die from radiation sickness later on.

Three days later, the U.S. dropped a second bomb, nicknamed "Fat Man," on Nagasaki, the blast alone killing as many as 80,000 more Japanese. On August 15, Emperor Hirohito announced his country's surrender. World War II was over.

'Mutually Assured Destruction'

Since 1945, many Americans have questioned whether the atomic bomb was worth the devastation it caused. It probably saved many thousands of American lives, forcing a quick end to the war. But it heralded a new world in which nations would have the power to annihilate one another as never before.

By 1949, the Soviet Union had its own bomb, leading to an "arms race" with the U.S. As more advanced missiles were developed, officials in both nations adopted a policy of "mutually assured destruction" (MAD). That meant neither side dared to strike the other for fear of being wiped out in return.

That principle was put to the test in 1962, when the Soviets placed nuclear missiles in Cuba, just 90 miles from...