Three days after Hiroshima, a second atomic bomb, a plutonium-based device named 'Fat Man,' was dropped on Nagasaki by the B-29 bomber 'Bockscar.' Nagasaki was the secondary target, chosen after the primary target, Kokura, was obscured by clouds. Simultaneously, the Soviet Union declared war on Japan and launched a massive invasion of Japanese-occupied Manchuria (Operation August Storm), fulfilling a promise made at the Yalta Conference. The combination of the atomic bombings and the entry of the Soviet Union—which removed any hope of a negotiated peace through Moscow—finally broke the stalemate within the Japanese government. Emperor Hirohito intervened, ordering the Supreme Council to accept the Allied terms, leading to Japan's announcement of surrender on August 15, 1945.