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 and in the presence of Adolf Hitler. It was a defensive military alliance that was eventually joined by Hungary, Romania, Slovakia, Bulgaria, and Yugoslavia. 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 Tsardom of Bulgaria, also referred to as the Third Bulgarian Tsardom, sometimes translated in English as the "Kingdom of Bulgaria", or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October 1908, when the Bulgarian state was raised from a principality to a tsardom.