On July 16, 1945, a team of scientists and engineers watched the first successful atomic bomb explosion at the Trinity test site in Alamogordo, New Mexico. The team, dubbed “The Manhattan Project,” had been secretly developing the weapon at the Los Alamos Laboratory during World War II. By the time it was ready, the Allies had already declared victory in Europe, but were still fighting in Japan.

Physicist J. Robert Oppenheimer, the director of the laboratory and so-called “father of the atomic bomb,” watched from afar that morning as the bomb released a mushroom cloud 40,000 feet high. His description of that moment has since become famous:

“I remembered the line from the Hindu scripture the Bhagavad-Gita,” he said. “‘Now I am become Death, the destroyer of worlds.’ I suppose we all thought that, one way or another.”

On August 6, the U.S. dropped the bomb on Hiroshima, Japan, wiping out 90 percent of the city and killing 80,000 people. Three days later, the U.S. killed 40,000 people in Nagasaki with another bomb. Tens of thousands more would die from radiation exposure. Japan surrendered a few days after the second bombing, ending World War II.

As details of the horrific destruction reached the Manhattan Project scientists, many began to question what they had done. In late October, Oppenheimer visited President Harry S. Truman, who had okayed the use of both bombs, to talk to him about placing international controls on nuclear weapons. Truman, worried about the prospect of Soviet nuclear development, dismissed him.

When Oppenheimer said he felt compelled to act because he had blood on his hands, Truman angrily told the scientist that “the blood is on my hands, let me worry about that.” He then kicked him out of the Oval Office, writes author Paul Ham in Hiroshima Nagasaki: The Real Story of the Atomic Bombings and Their Aftermath.

Ham isn’t convinced that Oppenheimer felt remorse specifically for the bombing of Japan, which the scientist may have viewed as a necessary evil. Rather, he thinks that Oppenheimer was more concerned about the devastation that future nuclear war could bring.

After the war, Oppenheimer took steps to prevent such a future. He began working with the U.S. Atomic Energy Commission to control the use of nuclear weapons. In 1949, when Truman approached the commission about creating a hydrogen bomb, Oppenheimer opposed it.

Despite his opposition, the U.S. developed an H-bomb and tested it in 1952. But Oppenheimer’s resistance ended up costing him his job. During the McCarthy era, the government stripped him of his job with the commission, citing his opposition to the hydrogen bomb as well as his purported Communist ties.

Oppenheimer’s blacklisting had more to do with his stance on the H-bomb than his Communist friends. Still, it created a scandal that followed him until his death in 1967. For decades afterwards, people continued to speculate about whether he was a Soviet spy.

Today, Oppenheimer is mostly remembered as a scientist who was persecuted for trying to address the moral problems of his creation. Though there have been some close calls, no country has used nuclear bombs as weapons since Hiroshima and Nagasaki. This means that, so far, we’ve been able to avoid the nuclear future Oppenheimer feared he’d already set in motion.