Short answers, pulled from the story.
Economic desperation and a fragile dependency on colonial financial investments drove radical military groups to seek new opportunities abroad. The world financial crisis left Japan in chronic malaise for three years before the invasion began.
The detonation occurred at around 10:20 pm on September 18, which served as a pretext for immediate military action. Fighting between Japanese Railway guards and Chinese soldiers ensued shortly after the blast.
By the end of September 19, the Japanese occupied Yingkou, Liaoyang, Shenyang, Fushun, Dandong, Siping, and Changchun. On September 21, the Japanese captured Jilin City, and they took Harbin on the 4th of February 1932.
The Lytton Commission delivered its findings in October 1932 stating that the Japanese puppet state of Manchukuo not be recognized. The report also called for the return of Manchuria to Chinese sovereignty, leading the Japanese government to withdraw from the League entirely.
American historian Louise Young described Japan from September 1931 to spring 1933 as gripped by war fever. Mainstream newspapers like the Asahi switched over to an aggressively militaristic editorial position while some pacifists wrote poems praising bushidō.