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Why did Japan sign the Tripartite Pact with Germany and Italy in 1940?

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Map showing the territorial expansion of the Japanese Empire from 1870 to 1942 across East and Southeast Asia and the Pacific. The map uses color codes to indicate the original Japanese territory in 1870, subsequent acquisitions, and the farthest extent of Japanese occupation and conquest by 1942, including parts of China, Korea, Manchuria, Southeast Asia, and numerous Pacific islands. An inset map displays the broader reach of Japanese control at its peak. — Empire of Japan The Empire of Japan, 1870–1942.

Japan signed the Tripartite Pact on September 27, 1940, primarily as a strategic move to deter the United States from entering World War II.12

By forming this defense alliance with Germany and Italy, Japan sought to solidify its position as the dominant power in Asia and secure its "new order" in the region. The pact included a provision requiring the signatories to assist one another with all political, economic, and military means if any of them were attacked by a power not then involved in the European War or the Sino-Japanese Conflict—a clause specifically aimed at the United States.23145

This alignment followed a period of deteriorating relations between Japan and the democratic powers, exacerbated by U.S. trade restrictions and embargoes intended to check Japanese expansionism. While the pact was intended to discourage American intervention, it ultimately failed to prevent the escalation of tensions that led to the attack on Pearl Harbor in 1941.2351