It was at _____ that the U.S. President told Stalin about the atomic bombYalta Casablanca Tehran Potsdam
The answer to this question is 4-Potsdam. The US President in question was Harry S. Truman. The United States first successfully tested an atomic bomb in New Mexico as the Potsdam Conference was beginning, and Truman strongly hinted to Stalin at the conference that the United States had an extraordinarily destructive new weapon. Stalin seemed unsurprised, perhaps because he had extensive knowledge of the Manhattan Project thanks to his spies. The announcement, however, encouraged the United States to take a firmer line than that taken by Roosevelt in earlier conferences with respect to the fate of Eastern Europe. Shortly after informing Stalin of the weapon, Truman issued the Potsdam Declaration, which demanded the unconditional surrender of Japan in the face of utter destruction by the new weapon.