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The Liáodōng Peninsula (simplified Chinese: 辽东半岛; traditional Chinese: 遼東半島; pinyin: Liáodōng bàndǎo) is a peninsula in the Liaoning Province of northeastern 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 commandries of Liaoxi (simplified Chinese: 辽西; traditional Chinese: 遼西) (West of the Liao River) and Liaodong during time of the Warring States.
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.
There are two seaports: Dalian, which lies midway along the peninsula at its narrowest point, and Port Arthur/Lushun (now part of the greater Dalian City), which is located at its southernmost point. Lüshun/Port Arthur has a large lake-like naturally-protected harbor and semi-sheltered outer roadstead making it very attractive to imperialist powers at the end of the nineteenth century. Dalny/Dairen/Dalian's harbor required greater investment, initially supplied by the Russians, but turned into a first class city under the Japanese period of administration (1905–1945). The two ports are about 25 miles (40 km) apart by rail, but about 40 nautical miles (70 km) apart by sea. Lüshun is 550 rail miles south of the Manchurian railroad hub city of Harbin on the historic Southern Manchurian Railway (today's China Far East Railway (CER), which construction was one of the underlying causes of the Russo-Japanese War).
Liaodong came under the rule of the Gojoseon kingdom when they took it from the native tribes. In the late 4th century B. C., the Chinese State of Yan invaded and conquered this region from Gojoseon. Later on various states such as the Han Dynasty, Gongsun Yuan, Cao Wei, Western Jin, Former Yan, Former Qin, Later Yan, Goguryeo, Tang Dynasty, Balhae, Khitan, Jurchen, 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 Empire which was exploited by colonial powers who extracted numerous concessions. The peninsula was ceded to Japan by the Treaty of Maguan of 17 April 1895 but this was rescinded after the Triple Intervention of 23 April 1895 by Russia, France and Germany.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 ruling 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. 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 given the lease for the Liaotung/Liaodong (Kwantung Leased Territory).