From Wikipedia

The Mukden Incident of September 18, 1931, also called the Manchurian Incident, occurred in southern Manchuria when a section of railroad, owned by Japan‘s South Manchuria Railway, near Mukden (today’s Shenyang) was blown up by Japanese junior officers.^[1] Imperial Japan‘s military accused Chinese dissidents of the act, thus providing a pretext for the Japanese annexation of Manchuria. The incident represented an early event in the Second Sino-Japanese War, although full-scale war would not start until 1937. In Chinese, this incident is referred to as the September 18 Incident (Chinese: 九?一八事变/九?一八事變) or Liutiaogou Incident (Chinese:柳條溝事變), or in Japanese as the Manchurian Incident (Kyūjitai: 滿洲事變, Shinjitai: 満州事変).

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The aim of Japanese junior officers in Manchuria was to provide a pretext that would justify Japanese military invasion and replace the Chinese government in the region with either a Japanese or a puppet one. They chose to sabotage a railway section in an area near Liutiao Lake (Traditional Chinese: 柳條湖). The fact was that the area had no official name and was not militarily important to either the Japanese or the Chinese. But it was only eight hundred meters away from the Chinese garrison of Beidaying (Traditional Chinese: 北大營), which was stationed by troops under the command of the “Young Marshal” Zhang Xueliang. The alleged Japanese plan was to attract Chinese troops by an explosion and then blame them for having caused it to provide a pretext for a formal Japanese invasion. In addition, to make the sabotage to appear more convincingly as a calculated Chinese attack on an essential transportation target ? thereby masking the Japanese action as a legitimate measure to protect a vital railway of industrial and economic importance ? the Japanese press labeled the site Liutiaogou (Traditional Chinese: 柳條”溝”) or Liutiaoqiao (Traditional Chinese: 柳條”橋”), which meant “Liutiao Ditch” and “Liutiao Bridge”, respectively, when in reality the site was a small railway section laid on an area of flat land. The choice to place the explosives at this site was to preclude the extensive reconstruction that would have been necessitated had the site truly been a railway bridge.

Colonel Itagaki Seishiro,^{[citation needed]} Lieutenant Colonel Kanji Ishiwara, Colonel Kenji Doihara, and Major Takayoshi Tanaka^[2] had laid complete plans for the incident by May 31, 1931. An important part of the scheme was to construct a swimming pool at the Japanese officers’ club in Mukden. This “swimming pool” was actually a concrete bunker for two 9.2-inch artillery pieces, which were brought in under complete secrecy.^[3]

The plan was executed when officers of the Shimamoto Regiment, which guarded the South Manchuria Railway, arranged for sappers to place explosives near the tracks, but far enough away to do no real damage. At around 10:20PM (22:20), September 18, the explosives were detonated. However, the explosion was minor and only a 1.5 meter section on one side of the rail was damaged. In fact, a train from Changchun passed by the site on this damaged track without difficulty and arrived at Shenyang at 10:30PM (22:30).