News of the successful test reached Truman, who was attending the final meeting of the “Big Three” Allied powers at Potsdam, Germany. Truman informed Soviet leader Joseph Stalin that the United States possessed “a new weapon of unusual destructive force.” On July 26 the Big Three issued an ultimatum, calling on Japan to surrender unconditionally or face “prompt and utter destruction.” When it became clear that no surrender was imminent, plans to use the bomb went into effect. Some within the Manhattan Project had argued for a demonstration explosion on an uninhabited site in the Pacific. This was considered but soon discarded, largely because of concerns that the demonstration bomb might not prompt sufficient reaction from the Japanese government. By this time, several dozen B-29 bombers had been modified to carry the weapons, and a staging base at Tinian, in the Mariana Islands, 1,500 miles (2,400 km) south of Japan, had been expanded into the largest airfield in the world.
On July 16, just hours after the successful completion of the Trinity test, the heavy cruiser USS Indianapolis left port at San Francisco with the gun assembly mechanism, roughly half of the U.S. supply of uranium-235, and several Los Alamos technicians. The remainder of the U.S. uranium-235 stockpile was flown to Tinian on transport planes. Upon the arrival of the Indianapolis at Tinian on July 26, assembly began on the bomb, dubbed Little Boy. The Indianapolis departed Tinian after the delivery, but it was sunk en route to the Philippines by the Japanese submarine I-58 on July 30. Hundreds of crew members who survived the torpedo attack died in the water while awaiting a rescue. The components of a second bomb, a plutonium device nicknamed Fat Man, were transported to Tinian by air. By August 2, 1945, both bombs had arrived at Tinian, and U.S. commanders were waiting only for a break in the weather to order the execution of Special Bombing Mission 13—an atomic attack on the Japanese home islands.