President Harry Truman told of Manhattan Project on this date in 1945

In the closing weeks of World War II’s European theatre, President Franklin D. Roosevelt died on April 12, 1945, of a cerebral hemorrhage at the age of 63. The responsibility of defeating Nazi Germany and the Empire of Japan and managing future post-war efforts fell upon his successor, Harry Truman.

Truman took the oath of office that day and, with the weight of nations on his shoulders, set about finishing the war.

Also that day, Secretary of War Henry L. Stimson, who in the past called then-Senator Truman and requested he cease inquiries into a Minneapolis factory that was tied to a secret weapons program, made quick mention to President Truman of a new weapon capable of destroying the entire world.

The Nazis would ultimately capitulate less than a month later. German Chancellor Adolf Hitler committed suicide in his bunker on April 30 during the Battle of Berlin. The remaining Nazi high command surrendered shortly thereafter, and Victory in Europe Day (V-E Day) was declared on May 8, leaving Japan as the allied powers’ only remaining foe.

Exactly two weeks after Truman ascended to the presidency, Secretary Stimson sent the Commander-in-Chief a brief, albeit important, note.

Truman would not see the note until the following morning, April 25, 1945. Stimson and Army General Leslie Groves were summoned to the White House and informed the president of the top secret research program to develop an atomic superweapon—the Manhattan Project.

Even though Germany had surrendered, Stimson and Groves, who was in charge of the project, imparted to Truman the significance of a weapon as a means of keeping the Soviets in check in the aftermath of the war.

Following the successful Trinity test of the first atomic bomb at Alamogordo on July 16, Truman gave approval to use the weapon against Japan. Prior to the Trinity test, the U.S. military and scientific members of the Manhattan Project discussed potential bombing targets. This “Targeting Committee” named five potential targets: Kokura, Hiroshima, Yokohama, Niigata, and Kyoto. However, Kyoto was later pulled from the list at Stimson’s insistence; Nagasaki was the replacement.

On August 6, the first atomic bomb was detonated over Hiroshima. Three days later, a second bomb was dropped on Nagasaki. It’s estimated that 130,000 to 215,000 people—mostly civilians—were killed in the bombings.

Japan surrendered on August 15, with the formal declaration taking place on September 2.

Secretary Stimson resigned on September 21. Two years later, he penned an opinion piece in Harper’s Magazine explaining the government’s rationale for developing and using atomic weapons.

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