The thirty divisions of the Royal Yugoslav Army were not equipped or prepared to meet the fifty-two invading German, Italian, and Hungarian divisions and the Bulgarian forces that invaded Macedonia. Lacking modern equipment and adequate mobility and firepower, the Yugoslav Army faced a surprise attack on several fronts by superior and heavily armored and mechanized forces. Yugoslav forces retreated rapidly to the center of the country, attempting to use the mountainous coastal areas as a base and to maintain lines of supply to Greece and the Allies. However, German forces captured the supreme command at Sarajevo on April 17, 1941, and Yugoslavia formally surrendered. Germany, Italy, Hungary, Bulgaria, and Albania annexed or occupied parts of the country.
A small group of officers led by Colonel Draza Mihajlovi refused to surrender and continued to resist the occupation from a base in western Serbia. They called themselves the Cetnik Detachments of the Yugoslav Army of the Fatherland. The Cetnici (sg. Cetnik--see Glossary) also represented the royal government in exile. They received a British military liaison officer and considerable amounts of British supplies and equipment. However, they avoided attacking the occupiers because they feared reprisals against the noncombatant population. The Cetnici believed their military actions could not influence the course of the war, and they waited instead for the Allies to defeat the Axis powers. They were later discredited in Yugoslavia as collaborators because of their unwillingness to resist.
The Communist Party of Yugoslavia (CPY) under Tito also refused to accept defeat. It remained inactive, however, until Germany attacked the Soviet Union on June 22, 1941. Through the Comintern (Communist Internal trans), the CPY received orders from the Soviet Union to resist the German occupation. Initially the military committees of the CPY collected arms and organized available manpower. Then they conducted small armed attacks and acts of sabotage against occupying Axis forces. They waged their military campaign without regard to the fate of civilians living under the occupation--often the occupiers executed large numbers of civilians in retaliation for attacks and sabotage. The difference in strategies and political views quickly brought the etnici and CPY forces into a state of civil war. The former unsuccessfully attempted to attack Tito's headquarters in November 1941.
The CPY military wing formally became the People's Liberation Army and Partisan Detachments of Yugoslavia (commonly known as the Partisans) on December 22, 1941. With approximately 80,000 fighters, the Partisans fought occupying forces, collaborators such as the Ustase (see Glossary) in Croatia, and their political opponents, the Cetnici. By the end of 1942, the Partisans had grown to 150,000 troops organized into two corps, three divisions, thirty-one brigades, and thirty-eight detachments. Axis occupation forces launched several major offensives to destroy the Partisans, but they failed in each case. Although the Partisans liberated some areas of the country, they generally avoided major engagements with superior forces.
Yugoslavia became an unanticipated theater of war for the Axis. Large German forces were forced to remain there to protect lines of supply to Greece and North Africa during the critical year of 1942. Nearly 600,000 Axis troops, thirty-eight divisions in all, were needed to control the country and thus were unavailable as reinforcements for the pivotal battles of El Alamein and Stalingrad. The occupation of Yugoslavia drained significant Axis manpower and resources from other theaters over a long period of time. Partisan pressure was a factor in Italy's withdrawal from the war in September 1943. When Italy's twenty divisions left Yugoslav territory, Germany had to commit even greater numbers of soldiers to maintain its position there. At maximum strength the German occupying army included twenty-six divisions.
By late 1943, the Partisans began to resemble a regular army. With captured or abandoned Italian arms, they armed 300,000 combatants in eight corps and twenty-six divisions. At the end of 1943, virtually all Allied military assistance was transferred from the Cetnici to the Partisans, whose operations had the potential of hastening the defeat of Germany. From then until the end of the war, the Partisans received over 100 tanks, 300 field guns, 2,000 mortars, 13,000 machine guns, and 130,000 rifles from Great Britain and the United States. The Soviet Union provided even larger numbers of guns, mortars, and machine guns.
As the Germans retreated from Greece through Yugoslavia and the Soviet Red Army advanced into Romania in 1944, the Partisans cleared most of the German troops from the country while simultaneously battling their domestic Ustase and Cetnik enemies. Tito flew to Moscow to meet Stalin and to coordinate Partisan and Red Army operations on Yugoslav territory. The Red Army wheeled north after entering the country and, together with the Partisans, liberated Belgrade on October 20, 1944. The Red Army pursued the retreating German forces from northeast Yugoslavia into Hungary, leaving the Partisans in control in Yugoslavia. The 800,000 troops of the People's Liberation Army officially became the Yugoslav People's Army (YPA) on March 1, 1945.
Yugoslavia suffered 1.7 million dead during the war, out of a total population of 15 million. Of these, over 300,000 were killed in action. Another 400,000 were wounded. Yugoslav sources claimed that the Partisans inflicted over 450,000 enemy casualties. The amount of Ustase and Cetnik casualties in that total is unknown.