Short answers, pulled from the story.
Colonel Seishirō Itagaki and Lieutenant Colonel Kanji Ishiwara independently devised plans to provoke Japan into invading Manchuria by staging a false flag attack. They chose to sabotage a rail section near Liutiao Lake without direct orders from Tokyo.
Explosives detonated at around 10:20 p.m. on the 18th of September 1931. The event is commemorated annually at 10:00 a.m. on the 18th of September across major cities in China including Heilongjiang Jilin Liaoning and Hainan.
The sabotage occurred on a small railway section laid on flat land near Liutiao Lake only eight hundred meters from the Chinese garrison of Beidaying. This area had no official name and sat close to troops under Young Marshal Zhang Xueliang.
Japanese Kwantung Army officers believed conflict would benefit Japan and aimed to create a pretext for formal invasion. Plans to conquer the Northeast accelerated as they noted Chinese military weaknesses following the Soviet Red Army victory over Zhang Xueliang's forces.
Following the explosion Japanese troops attacked seven thousand Chinese garrison and occupied major cities including Changchun Dandong and surrounding areas within five months. By March 1932 puppet state Manchukuo was established with former emperor China Puyi installed as head of state.