Soviet–Japanese War in the context of "Operation Downfall"

The surrender of the Empire of Japan in World War II was announced by Emperor Hirohito on 15 August and formally signed on 2 September 1945, ending the war. 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 United Kingdom and China, the United States called for the unconditional surrender of Japan in the Potsdam Declaration on 26 July 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 US and the UK at the Tehran and Yalta Conferences.

On 6 August 1945, at 8:15 am local time, the United States detonated an atomic bomb over the Japanese city of Hiroshima. Sixteen hours later, American president Harry S. Truman called again for Japan's surrender, warning them to "expect a rain of ruin from the air, the like of which has never been seen on this earth." Late on 8 August 1945, in accordance with the Yalta agreements, but in violation of the Soviet–Japanese Neutrality Pact, the Soviet Union declared war on Japan, and soon after midnight on 9 August 1945, the Soviet Union invaded the Japanese puppet state of Manchukuo. Hours later, the U.S. dropped a second atomic bomb on the Japanese city of Nagasaki.

On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during World War II. The aerial bombings killed 150,000 to 246,000 people, most of whom were civilians, and remain the only uses of nuclear weapons in an armed conflict. Japan announced its surrender to the Allies on 15 August, six days after the bombing of Nagasaki and the Soviet Union's declaration of war against Japan and invasion of Manchuria. The Japanese government signed an instrument of surrender on 2 September, ending the war.

In the final year of World War II, the Allies prepared for a costly invasion of the Japanese mainland. This undertaking was preceded by a conventional bombing and firebombing campaign that devastated 64 Japanese cities, including an operation on Tokyo. The war in Europe concluded when Germany surrendered on 8 May 1945, and the Allies turned their full attention to the Pacific War. By July 1945, the Allies' Manhattan Project had produced two types of atomic bombs: "Little Boy", an enriched uranium gun-type fission weapon, and "Fat Man", a plutonium implosion-type nuclear weapon. The 509th Composite Group of the U.S. Army Air Forces was trained and equipped with the specialized Silverplate version of the Boeing B-29 Superfortress, and deployed to Tinian in the Mariana Islands. The Allies called for the unconditional surrender of the Imperial Japanese Armed Forces in the Potsdam Declaration on 26 July 1945, the alternative being "prompt and utter destruction". The Japanese government ignored the ultimatum.

World War II officially ended in Asia on September 2, 1945, at 3:24 with the surrender of Japan on the USS Missouri. Before that, the United States dropped two atomic bombs on Japan, and the Soviet Union declared war on Japan, causing Emperor Hirohito to announce the acceptance of the Potsdam Declaration on August 15, 1945, which would eventually lead to the surrender ceremony on September 2.

After the ceremony, Japanese forces continued to surrender across the Pacific, with the last major surrender occurring on October 25, 1945, with the surrender of Japanese forces in Taiwan to Chiang Kai-shek. The American occupation of Japan lasted from the end of the war until April 28, 1952, when the Treaty of San Francisco came into effect.

The Soviet invasion of Manchuria, formally known as the Manchurian Strategic Offensive Operation or simply the Manchurian Operation (Маньчжурская операция) and sometimes, mainly in the West, as Operation August Storm, began on 9 August 1945 with the Soviet invasion of the Empire of Japan's puppet state of Manchukuo, which was situated in Japanese-occupied Manchuria. It was the largest campaign of the 1945 Soviet–Japanese War, which resumed hostilities between the Union of Soviet Socialist Republics and the Empire of Japan after almost six years of peace.

The invasion began hours before the atomic bombing of Nagasaki and 3 days after the atomic bombing of Hiroshima. The Soviet entry into this theater of the war and the defeat of the Kwantung Army were significant factors in the Japanese government's decision to surrender unconditionally on 15 August, as it became apparent that the Soviet Union had no intention of acting as a third party in negotiating an end of the war on conditional terms. The Kwantung Army officially surrendered on 16 August, although fighting continued until 2 September, when was firmed the Japanese Instrument of Surrender.

The Pacific War, sometimes called the Asia–Pacific War or the Pacific Theater, was the theater of World War II fought between the Japan and the Allies in East and Southeast Asia, the Pacific and Indian Oceans, and Oceania. It was geographically the largest theater of the war, including the Pacific Ocean theater, the South West Pacific theater, the Second Sino-Japanese War, and the brief Soviet–Japanese War, and included some of the largest naval battles in history. War between Japan and the Republic of China had begun in 1937, with hostilities dating back to Japan's invasion of Manchuria in 1931, but the Pacific War is more widely accepted to have begun in 1941, when the United States and United Kingdom were brought into the war, after being attacked by Japan.

Japan invaded French Indochina in 1940, and extended its control over the entire territory in July 1941. On 7–8 December 1941, Japan attacked the American naval base at Pearl Harbor in Hawaii; the U.S.-held Philippines, Guam, and Wake Island; and the British colonies of Malaya, Singapore, and Hong Kong, followed by a Japanese declaration of war. The Japanese achieved great success over the next 6 months, allying with Thailand and capturing the listed territories (except for Hawaii) in addition to Borneo, New Britain, the Dutch East Indies, Burma, the Solomon and Gilbert Islands, and parts of New Guinea. In May 1942, Japanese and Allied aircraft carriers fought at the Battle of Coral Sea, resulting in the retreat of a Japanese invasion force headed for Port Moresby. In June, Japan invaded the Aleutian Islands, and in the central Pacific was defeated at the Battle of Midway, considered a key turning point in the war. After this point, the Japanese experienced great difficulty replacing their losses in ships and aircraft as the U.S. produced ever increasing numbers of both.

The Invasion of the Kuril Islands (Russian: Курильская десантная операция, lit. 'Kuril Islands Landing Operation') was the World War II Soviet military operation to capture the Kuril Islands from Japan in 1945. The invasion, part of the Soviet–Japanese War, was decided on when plans to land on Hokkaido were abandoned. The successful military operations of the Red Army at Mutanchiang and during the invasion of South Sakhalin created the necessary prerequisites for invasion of the Kuril Islands.