Japanese imperialist ambition steadily increased during the 1920s, focusing on the complete conquest of Manchuria, but the powerful Chinese warlord Zhang Zuolin stood in Japan’s way. As a result, in 1928, the Japanese assassinated Zhang Zuolin by bombing his train. Imperial Japan’s military aggressions continued and culminated in the 1931 Mukden Incident, which was the explosion of a railway track. The Japanese had engineered this so-called act of sabotage, using it as a pretext to invade all of Manchuria, and immediately established the Puppet State of Manchukuo with PuYi, the last emperor of the Qing Dynasty as its sovereign.