The Manchuria Incident took place on September 18, 1931 in southern Manchuria when a section of railroad, owned by the Japanese company South Manchurian Railways, near Moukden (now Shenyang), was destroyed. This bombing attack was most seemingly planned by the Japanese army, because they feared a imminent unification of China under the Guomindang, which was perceived as a real threat to Japanese preeminence in the region. The Japanese army accused the Chinese of the alleged bombing attack, and justified the immediate attack on southern Manchuria, and then then established a puppet state of Manchshuria. This was the beginning of Japanese invasion on Manchuria.
This incident is not the only one. The same procedures as this incident were used in the late 1920s and when invading the rest of China in an attempt to justify the expansion of Imperial Japan.
In China, this incident is known as Liutiaogou incident.
After the Russo-Japanese War (1904-1905), Russian ruling of Manchuria was replaced by Imperial Japan. Until the establishment of the Imperial Headquarters in 1937, the Japanese Army had some independence in Manchuria and northern China from both the civilian government and the military authority in Japan.
At the time, Emperor Showa and his advisors were debating whether to conquer China militarily and establish colonial-style rule, or to subjugate China economically. Moreover, the Japanese government wanted to keep China fragmented so that it could deal to its advantage with the various Chinese factions, which were in open conflict with each other.
The Chinese policy at that time followed the first internal pacification, and internal resistance, and seemed to want to appease the Japanese.
The goal of the Japanese Military officials in Manchuria was to provide a pretext that could justify the Japanese military invasion and replacement of the Chinese government in the region with a Japanese or puppet government. So they chose to sabotage a section of railroad near Lake Liutiao. The plan was to see the explosion attract the attention of Chinese troops, and then blame them for it to give a pretext for a formal Japanese invasion. The military made the sabotage more convincing as a Chinese attack on a Japanese transportation target, and thus disguised their intervention as a legitimate measure to protect a railroad of industrial and economic importance.
Right after the explosion, the Japanese army attacked the nearby Chinese garrison and destroyed the troops that were stationed there for reason that all Japanese property must be protected from Chinese army. Some five hundred Japanese soldiers took part in attacking a Chinese garrison where over seven thousand soldiers were stationed.
Although whether the Japanese government was involved in this incident or not is not proven to this date, one thing that is certain is that the Japanese military stationed in Guandong area was prepared to proceed very swiftly to invade Manchuria and that its reaction was and perfectly disproportionate to the scale of the incident.