The Second Sino-Japanese War, known in China as the War of Resistance Against Japanese Aggression, was fought between the Republic of China and the Empire of Japan and its puppet states between 1937 and 1945, following a period of war localized to Manchuria that started in 1931. It is often regarded as the beginning of World War II in Asia, as the wars became heavily intertwined after Japan's entry into World War II. It was the largest Asian war in the 20th century.