Soon after arriving at the Potsdam Conference in July 1945, U.S. President Harry S. Truman received word that the scientists of the Manhattan Project had successfully detonated the world’s first nuclear device in a remote corner of the New Mexico desert.

On July 24, eight days after the Trinity test, Truman approached Soviet Premier Joseph Stalin, who along with Truman and British Prime Minister Winston Churchill (soon to be succeeded by Clement Attlee) made up the “Big Three” Allied leaders gathered at Potsdam to determine the post-World War II future of Germany.

According to Truman, he “casually mentioned” to Stalin that the United States had “a new weapon of unusual destructive force,” but Stalin didn’t seem especially interested. “All he said was that he was glad to hear it and hoped we would make ‘good use of it against the Japanese,’” Truman later wrote in his memoir, _Year of Decision_s.

For Truman, news of the successful Trinity test set up a momentous choice: whether or not to deploy the world’s first weapon of mass destruction. But it also came as a relief, as it meant the United States wouldn’t have to rely on the increasingly adversarial Soviet Union to enter World War II against Japan.

Truman never mentioned the words “atomic” or “nuclear” to Stalin, and the assumption on the U.S. side was that the Soviet premier didn’t know the exact nature of the new weapon. In fact, while Truman himself had first learned of the top-secret U.S. program to develop atomic weapons just three months earlier, after Franklin D. Roosevelt’s death, Soviet intelligence had begun receiving reports about the project as early as September 1941.

While Stalin didn’t take the atomic threat as seriously during wartime as some of his spies did—he had other problems on his hands, thanks to the German onslaught and occupation—Truman’s words at Potsdam made more of an impact than the president realized.

“We now know that Stalin immediately went to his subordinates and said, we need to get Kurchatov working faster on this,” says Gregg Herken, emeritus professor of U.S. diplomatic history at the University of California and the author of The Winning Weapon: The Atomic Bomb in the Cold War and Brotherhood of the Bomb. Igor Kurchatov was the nuclear physicist who headed up the Soviet atomic bomb project—the Soviet equivalent, in other words, of Manhattan Project mastermind J. Robert Oppenheimer.

On August 6, 1945, just days after the Potsdam Conference ended, the U.S. bomber Enola Gay dropped the uranium bomb known as “Little Boy” on the Japanese city of Hiroshima. Despite its devastating effects, Japan didn’t offer unconditional surrender right away, as the United States had hoped. Then on August 8, Soviet forces invaded Japanese-occupied Manchuria, violating an earlier non-aggression pact signed with Japan.

Herken argues that the Soviet invasion may have had at least as great an effect on Japanese morale as the first atomic bomb. “The last hope for the Japanese government, the peace faction, was that the Soviet Union might actually agree to negotiate a peace with the United States as a neutral party,” he explains. “But once the Soviets invaded Manchuria, it was clear that was not going to happen.”

On August 9, U.S. forces dropped “Fat Man,” a plutonium bomb, on Nagasaki. Together, the two bombs dropped in Japan would kill more than 300,000 people, including those who died instantly and those who perished from radiation and other lingering effects of the explosions.

Emperor Hirohito announced Japan’s unconditional surrender via radio address on August 15, bringing World War II to a close. In the peace negotiations at Yalta, as at Potsdam, the ideological gulf between the Soviet Union and its Western allies solidified, particularly when it came to the fate of Eastern Europe.

Even today, historians continue to disagree over whether or not the Truman administration made the decision to drop the atomic bomb for political reasons—namely, to intimidate the Soviet Union—rather than strictly military ones.

“The bomb was so top secret that there were no formal meetings about it, there was no official discussion about what to do, there wasn't the kind of decision-making process that we have with most kinds of policy,” says Campbell Craig, professor of international relations in the School of Law and Politics at Cardiff University and co-author of The Atomic Bomb and the Origins of the Cold War (with Sergey Radchenko). “So a lot of our opinions about what really drove the United States to drop the bomb is guesswork.”

Whatever the U.S. intention had been at Hiroshima and Nagasaki, Stalin certainly saw U.S. possession of the atomic bomb as a direct threat to the Soviet Union and its place in the post-war world—and he was determined to level the playing field. Meanwhile, thanks to atomic espionage, Soviet scientists were well on their way to developing their own bomb.

Some members of Truman’s administration would argue in favor of cooperation with the Soviets, seeing it as the only way to avoid a nuclear arms race. But an opposing view, articulated by State Department official George Kennan in his famous “Long Telegram” in early 1946, would prove far more influential, inspiring the Truman Doctrine and the “containment” policy toward Soviet and communist expansionism around the globe.

Later in 1946, during the first meeting of the United Nations Atomic Energy Commission (UNAEC), the United States presented the Baruch Plan, which called for the Soviets to share every detail of their atomic energy program—including opening their facilities to international inspectors—before the United States would share anything with them. Surprising no one, the Soviets rejected these terms.

“The Baruch Plan would have required the Soviets to basically surrender their sovereignty for them to have any share in atomic energy,” Herken says. “Stalin was the last person to want to do that.”

By 1949, all thoughts of cooperation were off the table: On August 29, the Soviets successfully tested their own nuclear device, producing a 20-kiloton blast roughly equal to the Trinity test. The nuclear arms race that would define the rest of the Cold War was on, as the two superpowers battled to see who could amass the most weapons of mass destruction, and figure out how to deploy them most effectively.

As Craig says, “The existence of the bomb forced the United States and the Soviet Union more quickly to reckon with one another than if the bomb hadn't existed.”