On September 18, 1931, a small charge exploded beside the tracks of the South Manchuria Railway near Mukden, now Shenyang. The damage was negligible and a train passed over the track minutes later. Japanese troops, who had placed the explosives, attacked the Chinese barracks at Beidaying the same night, and by the next morning they had taken the city.
It was a fabrication. Everyone knew it. The officers of the Kwantung Army who planned it, principally Itagaki Seishiro and Ishiwara Kanji, wanted a war in Manchuria and expected Tokyo to stop them. Tokyo did not. The cabinet in Tokyo ordered the army not to expand, the army ignored the order, and within five months Japanese forces had occupied the whole of northeast China, an area three times the size of Japan.
The reason it worked was that the chain of command did not run. Japanese army officers in the field answered to the general staff, not to the cabinet, and the general staff was sympathetic. China, meanwhile, was in the middle of a civil war between the Nationalists and the Communists, and Chiang Kai-shek's government chose to fight the Communists first and to take the Manchurian question to the League of Nations. That is what it did, and it was not enough.
The League sent the Lytton Commission, which took a year and reported in October 1932 that Japan had been attacked by nothing and had acted in self-defence against nothing. It recommended that Manchuria stay in China under a form of autonomy. Japan rejected the report and walked out of the League in March 1933, the first member ever to leave, which finished the League's credibility as anything but a debating society.
What followed was the puppet state of Manchukuo, declared in March 1932 with the last Qing emperor Puyi as its nominal ruler, and Japanese settlement and industrial development on a large scale. China never recognised it. The war that began on September 18 is usually counted as the start of the Second Sino-Japanese War, and the fighting continued, with a lull, until 1937 and then to 1945. September 18 is marked in China as National Humiliation Day.