Second Sino-Japanese War: The Chinese National Revolutionary Army begins the defense of Harbin.

The Second Sino-Japanese War was fought between the Republic of China and the Empire of Japan between 1937 and 1945, following a period of war localized to Manchuria that started in 1931. It is considered part of World War II, and often regarded as the beginning of World War II in Asia. It was the largest Asian war in the 20th century and has been described as The Asian Holocaust, in reference to the scale of Japanese war crimes against Chinese civilians, similar to the European ones. It is known in China as the War of Resistance Against Japanese Aggression.

The Republic of China (ROC) began on 1 January 1912 as a sovereign state in mainland China following the 1911 Revolution, which overthrew the Manchu-led Qing dynasty and ended China's imperial history. From 1927, the Kuomintang (KMT) reunified the country and initially ruled it as a one-party state with Nanjing as the national capital. In 1949, the KMT-led government was defeated in the Chinese Civil War and lost control of the mainland to the Chinese Communist Party (CCP). The CCP established the People's Republic of China (PRC) while the ROC was forced to retreat to Taiwan; the ROC retains control over the Taiwan Area, and its political status remains disputed. The ROC is recorded as a founding member of both the League of Nations and the United Nations, and previously held a permanent seat on the United Nations Security Council until 1971, when the PRC took China's seat in the United Nations General Assembly Resolution 2758. It was also a member of the Universal Postal Union and the International Olympic Committee. The ROC claimed 11.4 million km² (4.4 million sq mi) of territory, and its population of 541 million in 1949 made it the most populous country in the world.

The National Revolutionary Army served as the military arm of the Chinese Nationalist Party from 1924 until 1947.