On September 27, 1940, the Axis powers are formed as Germany, Italy, and Japan become allies with the signing of the Tripartite Pact in Berlin. The Pact provided for mutual assistance should any of the signatories suffer attack by any nation not already involved in the war. This formalizing of the alliance was aimed directly at “neutral” America–designed to force the United States to think twice before venturing in on the side of the Allies.
The Pact also recognized the two spheres of influence. Japan acknowledged “the leadership of Germany and Italy in the establishment of a new order in Europe,” while Japan was granted lordship over “Greater East Asia.”
A footnote: There was a fourth signatory to the Pact-Hungary, which was dragged into the Axis alliance by Germany in November 1940.