The Tripartite Pact, also known as the Berlin Pact, was an agreement between Germany, Italy, and Japan signed in Berlin on 27 September 1940 by, respectively, Joachim von Ribbentrop, Galeazzo Ciano and Saburō Kurusu. It was a defensive military alliance that was eventually joined by Hungary (20 November 1940), Romania (23 November 1940), Bulgaria (1 March 1941) and Yugoslavia (25 March 1941) as well as by the German client state of Slovakia (24 November 1940). Yugoslavia's accession provoked a coup d'état in Belgrade two days later. Germany, Italy and Hungary responded by invading Yugoslavia. The resulting Italo-German client state, known as the Independent State of Croatia, joined the pact on 15 June 1941.

The Tripartite Pact was directed primarily at the United States. Its practical effects were limited since the Italo-German and Japanese operational theatres were on opposite sides of the world, and the high contracting powers had disparate strategic interests. As such the Axis was only ever a loose alliance. Its defensive clauses were never invoked, and signing the agreement did not oblige its signatories to fight a common war per se.