The Liaodong Peninsula (simplified Chinese: 辽东半岛; traditional Chinese: 遼東半島; pinyin: Liáodōng Bàndǎo) is a peninsula in Liaoning Province of Northeast China, historically known in the West as Southeastern Manchuria. Liaodong (formerly spelled Liaotung) means "East of the Liao River"; referring to the Liao River which divided the Yan commanderies of Liaoxi (simplified Chinese: 辽西; traditional Chinese: 遼西) (West of the Liao River) and Liaodong during time of the Warring States period.
It forms the southern part of a mountain belt that continues northward in the Changbai Mountains. The part of the mountain range on the peninsula is known as the Qianshan Mountains, named after Qian Mountain in Anshan, which includes Dahei Mountain in Dalian.
Liaodong came under the rule of the Gojoseon kingdom which emerged in the region. In the late 4th century BC, the Chinese State of Yan invaded and conquered this region from Gojoseon. Later on various states and dynasties such as the Han Dynasty, Gongsun Yuan, Cao Wei, Western Jin, Former Yan, Former Qin, Later Yan, Goguryeo, Tang Dynasty, Balhae, Liao Dynasty, Jin dynasty (1115–1234), Yuan Dynasty, Ming Dynasty and Qing Dynasty ruled Liaoning.
The peninsula was an important area of conflict during the First Sino-Japanese War (1894–1895), which the Japanese won. Defeat precipitated decline in the Chinese Qing dynasty which was exploited by colonial powers who extracted numerous concessions. The peninsula was ceded to Japan, along with Taiwan and Penghu, by the Treaty of Shimonoseki of 17 April 1895. However the ceding of Liaodong peninsula was rescinded after the Triple Intervention of 23 April 1895 by Russia, France and Germany. In the aftermath of this intervention, the Russian government pressured the Qing dynasty to lease Liaodong and the strategically important Lüshunkou (Port Arthur) for use by the Russian Navy.
As in the First Sino-Japanese War the Liaodong peninsula was the scene of major fighting in the Russo-Japanese War (1904–1905). As a consequence of the Treaty of Portsmouth (5 September 1905), which ended the Russo-Japanese War, both sides agreed to evacuate Manchuria and return its sovereignty to China, but Japan was able to coerce a lease for the Liaotung/Liaodong, establishing the Kwantung Leased Territory.