Which event marked Japan's aggression in East Asia before World War II?

The Invasion of Manchuria in 1931 is a significant event that marked Japan's aggression in East Asia prior to World War II. This invasion exemplified Japan's imperial ambitions as it sought to expand its territory and influence in the region. Following the invasion, Japan established a puppet state called Manchukuo, which demonstrated its desire to control resources and enhance its strategic position in Asia.

This event is crucial because it not only set the stage for further Japanese expansionism but also served as an early indicator of the aggressive militarism that would characterize Japan's foreign policy leading up to the war. The invasion violated international agreements and prompted widespread condemnation, yet it laid the groundwork for Japan’s subsequent military actions in China and other parts of East Asia.

In contrast to the other events listed, which either occurred later in the context of World War II or were responses to earlier aggressions, the Invasion of Manchuria was one of the first acts of aggression that illustrated Japan's intentions and foreshadowed its role in the broader conflict of the war.