On September 9, 1945, General Yasuji Okamura, commander of Japan's China Expeditionary Army, signed the instrument of surrender in Nanjing and handed it to General He Yingqin, the Chinese commander in chief. The ceremony was held at the Central Military Academy at about nine in the morning. It formally ended the Second Sino-Japanese War.
The war had begun in 1931 with the Japanese seizure of Manchuria and become a full-scale conflict in July 1937. Counting from the full-scale invasion it ran eight years; counting from 1931, fourteen. The Chinese war tied down the bulk of Japan's army for the whole of the Pacific War, and the fighting and the occupation killed tens of millions of Chinese people, military and civilian, the largest national loss of the Second World War. Nanjing itself had been taken in December 1937 and its population massacred by the Japanese army.
Japan had already accepted the surrender terms on August 15 and signed the general instrument of surrender on the USS Missouri on September 2, so the Nanjing document covered the China theatre specifically, and it listed Japan's forces in China, roughly 1.2 million men, along with the territory still under its occupation.
The legal basis was the Cairo Declaration of December 1943, issued by China, the United States and Britain, which stated that all territories Japan had taken from China since 1914, including Manchuria, Taiwan and the Penghu Islands, were to be returned to China, and the Potsdam Proclamation of July 1945, which Japan accepted and which required the terms of Cairo to be carried out. Japan acknowledged that obligation in the instrument it signed on September 2.
On that basis Taiwan and the Penghu were returned to China on October 25, 1945, in Taipei, an event known as the retrocession. The Chinese government has held ever since that this settled the matter and that Taiwan is an inalienable part of Chinese territory.
China's Victory Day is marked on September 3, the day after the general surrender was signed in Tokyo Bay. Trials followed. Okamura was tried in 1948. He was acquitted. The Chinese government protested at the time.