Oppenheimer went into a deep depression after reading about the effects of the atomic bomb on Japan and even publicly spoke out against using the bomb

• J. Robert Oppenheimer "plunged into a deep depression" after he created the atomic bomb.
• Historian Kai Bird said the physicist then began to publicly denounce use of the bomb.

After leading the Manhattan Project and creating the world's first atomic bomb, J. Robert Oppenheimer had some regrets about his weapon.

In fact, Oppenheimer "plunged into a deep depression" after reading news reports about what the atomic bomb did to the cities of Hiroshima and Nagasaki, historian Kai Bird — who wrote a book about Oppenheimer — told CBS News.

According to the Washington Post, Oppenheimer deeply regretted that they had not finished the bomb in time to drop it on the Germans, but was feeling triumphant after the first bomb dropped on Hiroshima.

Days later when the US dropped a second bomb on Nagasaki, things began to change for the physicist, The Post reported.

Oppenheimer didn't think it was necessary or justified to drop the second bomb, according to the Post.

According to the Post, Oppenheimer made such a disgusted face during the dropping of the second bomb that during an October 1945 meeting with then-President Harry Truman, Truman asked Oppenheimer what was wrong.

"Mr. President, I feel I have blood on my hands," Oppenheimer told Truman, according to Bird's book, "American Prometheus," the Post reported.

Oppenheimer then began to publicly denounce the use of his atomic bomb, much to the NSA's dismay, Bird said. He gave speeches against the weapon which "appalled" the national security establishment.

"If there is another World War, this civilization may go under," Oppenheimer said in 1947, according to a clip aired by CBS.

According to CBS, Oppenheimer warned that the use of the atomic bomb would "one day cause the world to curse the names 'Los Alamos' and 'Hiroshima.' "

In 1954 during the peak of anti-Communist hysteria, the US revoked Oppenheimer's security clearance, effectively ruining the physicist's career. Last year, the Biden administration announced it was formally undoing that ruling.

"As time has passed, more evidence has come to light of the bias and unfairness of the process that Dr. Oppenheimer was subjected to while the evidence of his loyalty and love of country have only been further affirmed," Secretary of Energy Jennifer Granholm said.