The Japanese army invaded Manchuria in 1931. The opening move came on September 18, when Japan’s Kwantung Army used the Mukden Incident as a pretext to launch military operations in northeastern China.
Japanese officers staged a small explosion beside the South Manchuria Railway near Mukden, now Shenyang, and blamed Chinese forces. Although the blast caused little damage, Japanese troops quickly seized Mukden and expanded their campaign across Manchuria. The invasion was conducted without authorization from Japan’s civilian government in Tokyo.
By February 1932, Japan had occupied the region and established Manchukuo, a Japanese-controlled puppet state headed nominally by former Qing emperor Puyi. The League of Nations condemned the seizure after the Lytton Commission investigated, but Japan rejected the criticism and left the organization in 1933. A common mix-up is to date the event to 1932, the year the conquest was completed and Manchukuo was established; the invasion itself began in 1931.