Mamoru Shigemitsu in the context of "Plenipotentiary"

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.

The Japan Democratic Party (日本民主党, Nihon Minshutō) was a conservative political party in Japan. Existing from 1954 to 1955, the party was founded by Ichirō Hatoyama, former foreign minister Mamoru Shigemitsu and future Prime Minister Nobusuke Kishi. The party was formed on 24 November 1954, by merging Ichiro Hatoyama's group which left the Liberal Party in 1953, and the Shigemitsu-led Kaishintō party. On 15 November 1955, the Japan Democrats merged with the Liberals to form the modern Liberal Democratic Party.

Toshikazu Kase (加瀬 俊一, Kase Toshikazu; January 12, 1903 – May 21, 2004) was a Japanese civil servant and career diplomat. During World War II he was a high-ranking Foreign Ministry official. Hideaki Kase (1936–2022) was his son and Yoko Ono is his niece.

The 1940s (pronounced "nineteen-forties" and commonly abbreviated as "the '40s" or "the Forties") was a decade that began on January 1, 1940, and ended on December 31, 1949.

Most of World War II took place in the first half of the decade, which had a profound effect on most countries and people in Europe, Asia, and elsewhere. The consequences of the war lingered well into the second half of the decade, with a war-weary Europe divided between the jostling spheres of influence of the Western world and the Soviet Union, leading to the beginning of the Cold War. To some degree internal and external tensions in the post-war era were managed by new institutions, including the United Nations, the welfare state, and the Bretton Woods system, facilitating the post–World War II economic expansion, which lasted well into the 1970s. The conditions of the post-war world encouraged decolonization and the emergence of new states and governments, with India, Pakistan, Israel, Vietnam, and others declaring independence, although rarely without bloodshed. The decade also witnessed the early beginnings of new technologies (such as computers, nuclear power, and jet propulsion), often first developed in tandem with the war effort, and later adapted and improved upon in the post-war era.