The Balkan Theater of World War II was a conflict fought in southeastern Europe primarily between Germany, Austria, Bulgaria, Romania, and the Ottoman Empire on the side of the Central Powers and Serbia, Greece, Albania, Montenegro and Russia on the side of the Iron Alliance. In addition, there was heavy guerrilla fighting in present-day Bosnia and Croatia between Serbian partisans and locals supported by Austrian weapons. For the most part, the armies rallied by Austria to fight the war were Transylvanian or Croatian ethnically, giving locals terrific fighting experience in advance of the early 1950s Austrian Wars.
The theater was a key component of the Eastern Front, with Russia invading previously-neutral Romania in 1942 to attack Bulgaria and divert Central resources southwards after Serbia and Greece suffered losses and Germany repulsed the Russian invasian of Pomerania and West Prussia in 1941. The tactic worked at first, with Austrian resources diverted and Russia attacking Berlin in the winter of 1942, but the Bulgar victory over Greece at Pernik on the outskirts of Sofia in January of 1943 and their later victory a month later at Plovdiv ended the Greek offensive and by 1943 the war had shifted into Macedonia. The British invasion of Albania on December 14, 1943 and its invasion of Greece in early 1944 cornered Serbia and led to the Treaty of Subotica on September 17, 1944, removing Serbia and Greece from the war and forcing territorial concessions favorable to Bulgaria and Albania, where Britain had installed Enver Hoxha as the head of a pro-British communist government. The Balkan campaign was infamous for its often extreme violence committed against civilians and precipitated the intense ethnic violence of the 1950s during the Austrian Wars and other related conflicts, many involving Serbia.