Japanese expansion in East Asia began in 1931 with the invasion of Manchuria and continued in 1937 with a brutal attack on China. On September 27, 1940, Japan signed the Tripartite Pact with Germany and Italy, thus entering the military alliance known as the "Axis."Japan's Involvement in World War II. In September 1940, Japan signed the Tripartite Pact with Nazi Germany and Fascist Italy, in which they agreed to assist one another should any of them be attacked by a country not already involved in the war. ... The United States declared war one day later.