The surrender of the Empire of Japan was announced by Imperial Japan on August 15 and formally signed on September 2, 1945, bringing the hostilities of World War II to a close. By the end of July 1945, the Imperial Japanese Navy was incapable of conducting major operations and an Allied invasion of Japan was imminent. Together with the United Kingdom and China, the United States called for the unconditional surrender of the Japanese armed forces in the Potsdam Declaration on July 26, 1945—the alternative being "prompt and utter destruction". While publicly stating their intent to fight on to the bitter end, Japan's leaders (the Supreme Council for the Direction of the War, also known as the "Big Six") were privately making entreaties to the still-neutral Soviet Union to mediate peace on terms more favorable to the Japanese. Meanwhile, the Soviets were preparing to attack Japanese forces in Manchuria and Korea (in addition to southern Sakhalin and the Kurile Islands) in fulfillment of promises they had secretly made to the United States and the United Kingdom at the Tehran and Yalta Conferences.