Bulgaria's motives for entering World War II were once again based on irredentism, coupled with almost total economic dependence on Germany. Once more it hoped to regain the lands of Thrace and Macedonia, which were lost after the Treaty of San Stefano was reversed by the Congress of Berlin. The lesson of the two subsequent Balkan wars and World War I had fallen on deaf ears. Bulgaria was still estranged from its Balkan neighbors and once more was being courted by the former ally of World War I, Germany. Germany, again realizing Bulgaria's territorial aspirations, hoped to bribe the Bulgarian leadership with southern Dobrudzha, which was eventually ceded to Bulgaria in 1940.
In December 1941 Bulgaria placed herself squarely on the German side by declaring war on Great Britain and the United States and joining the Rome-Berlin Axis. This alignment, which derived primarily from Bulgaria's irredentist policy, was given further force by dislike of the British, who were held to blame by the Bulgarians for the loss of Macedonia to Yugoslavia and Greece.
Despite the declaration of war against Great Britain and the United States, Bulgaria refused throughout World War II to declare war on the Soviet Union. The Russians, unlike the British and Americans, were popular with the Bulgarian people. They were still remembered for their assistance to the Bulgarians in the past and were viewed by the people as their liberators from Turkish rule. Not only did Bulgaria refuse to declare war on its former liberator, but it also refused to make its army available to Adolf Hitler for his eastern campaign. When Germany declared war on Russia, Bulgaria continued to retain neutrality toward, and to maintain diplomatic relations with, the Soviet Union.
In the early stages of the war, before Bulgaria had declared war on the Allies, it had already begun to regain some of the land lost during the Balkan wars and World War I. Southern Dobrudzha, which had been ceded to Romania in 1913, reverted to Bulgaria by August 1940. In the spring of 1941, supporting Germany against Yugoslavia and Greece, Bulgaria regained Macedonia and part of Greek Thrace. When Bulgaria was rewarded with these lands by the Nazis, Bulgarians perceived their gains as a "historical national unification." By 1941 Yugoslavia was overrun, and some of its territories were taken by Italy, Hungary, and Bulgaria. Italy received Montenegro, Hungary took part of northern Yugoslavia, and Bulgaria gained, in addition to the much-prized Macedonia, the frontiers of southeastern Serbia. The Bulgarians at this point were once again approaching the frontiers that had been established by the Treaty of San Stefano.