The surrender of Imperial Japan was announced by Japanese Emperor Hirohito 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 (IJN) was incapable of conducting major operations and an Allied invasion of Japan was imminent. Together with the British Empire 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 publicly neutral Soviet Union to mediate peace on terms more favorable to the Japanese. While maintaining a sufficient level of diplomatic engagement with the Japanese to give them the impression they might be willing to mediate, the Soviets were covertly preparing to attack Japanese forces in Manchuria and Korea (in addition to South Sakhalin and the Kuril Islands) in fulfillment of promises they had secretly made to the United States and the United Kingdom at the Tehran and Yalta Conferences.