By 1939 Bulgaria had moved inexorably into the fascist sphere of Germany and Italy. The country was tied to the former for economic reasons and because Germany promised territorial revision for Bulgaria, and to the latter because Boris was married to the daughter of King Victor Emmanuel III of Italy. In the late 1930s, Bulgaria continued to seek rapprochement with Yugoslavia a friendship treaty was signed in 1937, and a renunciation of armed intervention in 1938. When Germany took the Sudetenland from Czechoslovakia in 1938, it ended the anti-German Little Entente alliance of Yugoslavia, Czechoslovakia, and Romania and pushed Yugoslavia closer to Bulgaria. When World War II began in September 1939 with the German invasion of Poland, Bulgaria declared neutrality, but this position was inevitably altered by big-power relationships.

The Nazi-Soviet alliance of 1939 improved Bulgaria's relations with the Soviet Union, which had remained cool, and yielded a Bulgarian-Soviet commercial treaty in 1940. The pro-Western Bulgarian Prime Minister Georgi Kioseivanov was deposed that year in favor of pro-German Bogdan Filov, who reduced cultural ties with the West and instituted a Nazi-type youth league. Meanwhile, Boris strove to maintain neutrality, rejecting Soviet treaty offers in 1939 and 1940. Boris also rejected membership in the Balkan Entente and in a proposed Turkish-Yugoslav-Bulgarian defense pact, because such moves would anger Italy, Germany, the Soviet Union, or all three. Under pressure from Hitler, Romania ceded southern Dobruja to Bulgaria by the Treaty of Craiova in 1940. Needing Bulgaria to anchor its Balkan flank, Germany increased diplomatic and military pressure that year. The massing of German troops in Romania prior to invading Greece removed all remaining flexibility aware that German troops would have to pass through Bulgaria to reach Greece, Bulgaria signed the Tripartite Pact with Germany and Italy in March 1941.

Data as of June 1992