On September 23, 1940, the Tripartite Pact is signed by representatives for Germany, Italy, and Japan. With the pact, the three nations agreed to recognize each other and their establishment of a new political order within their regions (Europe for Germany and Italy, Greater East Asia for Japan). The pact also called for the three nations to assist in the conquest of their respective regions and to assist each other should a foreign power not then involved in the European or East Asian wars attacked/declared war on one or more members. This more or less meant that the signatories had agreed to come to each other's aid in the event of the United States entering the war.

Other nations would later join the pact, including Bulgaria, Romania, Hungary, the Independent State of Croatia, and the Slovak Republic. The Kingdom of Yugoslavia would also sign the Tripartite Pact; however, a military coup overthrew the Yugoslavian king two days later, replaced him with his son, and revoked the agreement, an action that provoked an invasion by Nazi Germany.

Following the American entry into the war, the original three members of the Tripartite Pact (Germany, Italy, and Japan) added a further agreement that none would make peace with either the United States or Great Britain without the inclusion and agreement of the other two.