Franklin Delano Roosevelt may not be the first president Americans associate with the atomic bomb. That distinction usually goes to his wartime successor, Harry Truman. But it was FDR, anxious about Germany beating the World War II Allies to the bomb, who green-lit the weapon’s development.

In August 1945, Truman authorized the U.S. military to attack Japan with the fearsome new weapon. The subsequent attacks on Hiroshima and Nagasaki killed hundreds of thousands of people, and led to the surrender of Japan that month. Incredibly, Truman made the decision to use the nuclear bomb only four months after learning that the United States was building it.

That’s because Roosevelt never told Truman about the Manhattan Project’s top-secret bomb-making efforts—even after Truman became FDR’s fourth-term vice president in January 1945. Although Truman was ultimately responsible for deciding whether to deploy that weapon, it was FDR who vaulted America, and the world, into the nuclear age.

Scientists working in Germany and Switzerland discovered nuclear fission in December 1938,  and the discovery quickly prompted international discussion among scientists about whether nuclear fission could be used to develop a new energy source or weapon.

“It was immediately obvious to good physicists everywhere that this reaction would potentially be the basis for a new weapon of unsurpassed destructive capacity,” says Richard Rhodes, author of The Making of the Atomic Bomb.

Worried about the possibility of Nazi Germany developing a nuclear bomb, the Hungarian physicist and Jewish refugee Leo Szilard helped convince Albert Einstein, another Jewish physicist who’d fled Nazi Germany, to send a letter to President Roosevelt.

In the letter, Einstein warned that Germany could try to gather enough uranium to create a bomb so powerful it could destroy a whole port by itself. Einstein sent the letter through an intermediary in August 1939, and by the time it reached FDR that October, Adolf Hitler had invaded Poland and WWII had begun.

Even though the U.S. had not yet joined the war, Einstein’s letter prompted FDR to convene the Advisory Committee on Uranium. The next year, he approved the creation of the National Defense Research Committee, which was superseded in 1941 by the Office of Scientific Research and Development. These groups marked the tentative beginning of the U.S. nuclear program.

The big turning point came in the summer of 1941, when scientists on Britain’s MAUD Committee released a report outlining a feasible plan for building a nuclear bomb. Vannevar Bush, head of the Office of Scientific Research and Development, met with FDR in October to discuss the report with him. In response, FDR told Bush to begin research and development on a nuclear bomb, and that he would find a way to secure funding to build it.

The Japanese attack on Pearl Harbor on December 7, 1941 ushered the United States into the war, giving new urgency to the country’s quest to build a nuclear weapon. A month after this attack, FDR officially approved the creation of the Manhattan Project. The secret U.S. program to build the world’s first atomic bomb would end up costing $2.2 billion and employing 130,000 people, not all of whom knew what they were working on.

A big factor driving the creation of the Manhattan Project was the fear that Nazi Germany might create a nuclear bomb first. However, German attempts at nuclear programs never got very far. By 1944, Germany was losing the war, and scientists in the United States and Britain (which was now collaborating with the Manhattan Project) began to worry more about the impact that a potential U.S. bomb would have on the world.

The Danish physicist Niels Bohr, who worked on the Manhattan Project on behalf of Britain, was one of the most vocal of these scientists. In August 1944, he met with FDR to discuss his fears that the bomb could create a nuclear arms race in which countries would gain the power to annihilate each other. He suggested there should be an international plan for controlling nuclear weapons after the war.

When FDR met with Prime Minister Winston Churchill the next month, they discussed whether their countries should inform the world that they were working on a nuclear bomb. Churchill, who had also met with Bohr and remained unmoved by his concerns, strongly disagreed with this idea. In a memorandum signed at this meeting, FDR and Churchill agreed to keep the bomb a secret.

In March 1945, Einstein sent a second letter—again at the urging of Szilard—to FDR. Szilard,  who was working on the Manhattan Project in the United States, was one of the scientists increasingly alarmed about how nuclear weapons might change the world. In the letter, Einstein wrote of Szilard’s concern that there wasn’t enough communication between the scientists working on the bomb and the government officials who would decide how to use it. He urged FDR to meet with Szilard so the physicist could discuss his concerns.

To make sure FDR didn’t miss the message, Einstein sent a copy of the letter to First Lady Eleanor Roosevelt. She scheduled a meeting between Szilard and FDR that would take place in May. But the meeting never happened, because on April 12, the president died of a stroke.

When Harry Truman was a U.S. senator in 1943, he’d tried to investigate the government’s spending on the Manhattan Project. But it wasn’t until Truman succeeded FDR as president in April 1945 that he finally learned the project was building the world’s first nuclear weapon.

One thing FDR and Churchill had already discussed before Roosevelt’s death was which country they might use the bomb against. Their signed agreement in September 1944 stated that “when a ‘bomb’ is finally available, it might perhaps, after mature consideration, be used against the Japanese, who should be warned that this bombardment will be repeated until they surrender.”

But in terms of actual plans for using the bomb, and handling nuclear weapons going forward, “Roosevelt had left [Truman] no documentation or policy directives,” says Bruce Cameron Reed, a professor emeritus of physics at Alma College and author of Manhattan Project: The Story of the Century.

Secretary of War Harry Stimson had discussed a few possibilities of how to use the bomb with FDR. One idea: Provide a demonstration of the bomb’s power first, and then a warning that the United States would use the bomb on Japan if it didn’t surrender. Truman considered this strategy, but ultimately decided against it.

Because FDR died before the Manhattan Project completed the atomic bomb, it isn’t clear if he understood how much power the weapon would actually have. In fact, this was something that the scientists working on the bomb only really began to understand during the later stages of production. Roosevelt died before seeing the destruction the weapon unleashed—and the nuclear arms race that developed as a result.