World War II in the Kingdom of Yugoslavia began on 6 April 1941, when the country was invaded and swiftly conquered by Axis forces and partitioned among Germany, Italy, Hungary, Bulgaria and their client regimes. Shortly after Germany attacked the USSR on 22 June 1941, the Comintern issued directives to all communist parties to come to the Soviet Union's aid, following which the communist-led Yugoslav Partisans launched a guerrilla liberation war fighting against the Axis forces and their locally established puppet regimes, including the Axis-allied Independent State of Croatia (NDH) and the Government of National Salvation in the German-occupied territory of Serbia. This was dubbed the National Liberation War and Socialist Revolution in post-war Yugoslav communist historiography. Simultaneously, a multi-side civil war was waged between the Yugoslav communist Partisans, the Serbian royalist Chetniks, the Axis-allied Croatian Ustaše and Home Guard, Serbian Volunteer Corps and State Guard, Slovene Home Guard, as well as Nazi-allied Russian Protective Corps troops.

Both the Yugoslav Partisans and the Chetnik movement initially resisted the Axis invasion. However, after 1941, Chetniks extensively and systematically collaborated with the Italian occupation forces until the Italian capitulation, and thereon also with German and Ustaše forces. The Axis mounted a series of offensives intended to destroy the Partisans, coming close to doing so in the Battles of Neretva and Sutjeska in the spring and summer of 1943.

Despite the setbacks, the Partisans remained a credible fighting force, with their organisation gaining recognition from the Western Allies at the Tehran Conference and laying the foundations for the post-war Yugoslav socialist state. With support in logistics and air power from the Western Allies, and Soviet ground troops in the Belgrade offensive, the Partisans eventually gained control of the entire country and of the border regions of Trieste and Carinthia. The victorious Partisans established the Federal People's Republic of Yugoslavia.

The conflict in Yugoslavia had one of the highest death tolls by population in the war, and is usually estimated at around one million, about half of whom were civilians. Genocide and ethnic cleansing was carried out by the Axis forces (particularly the Wehrmacht) and their collaborators (particularly the Ustaše and Chetniks), and reprisal actions from the Partisans became more frequent towards the end of the war, and continued after it.

Background Prior to the outbreak of war, the government of Milan Stojadinović (1935–1939) tried to navigate between the Axis powers and the imperial powers by seeking neutral status, signing a non-aggression treaty with Italy and extending its treaty of friendship with France. At the same time, the country was destabilized by internal tensions, as Croatian leaders demanded a greater level of autonomy. Stojadinović was sacked by the regent Prince Paul in 1939 and replaced by Dragiša Cvetković, who negotiated a compromise with Croatian leader Vladko Maček in 1939, resulting in the formation of the Banovina of Croatia.

However, rather than reducing tensions, the agreement only reinforced the crisis in the country's governance. Groups from both sides of the political spectrum were not satisfied: the pro-fascist Ustaše sought an independent Croatia allied with the Axis; Serbian public and military circles preferred alliance with Britain and France, while the then-banned Communist Party of Yugoslavia saw the Soviet Union as a natural ally.

Following the fall of France in June 1940, Yugoslavia's Regent Prince Paul and his government saw no way of saving the Kingdom of Yugoslavia except through accommodation with the Axis powers. Although Germany's Adolf Hitler was not particularly interested in creating another front in the Balkans, and Yugoslavia itself remained at peace during the first year of the war, Benito Mussolini's Italy had invaded Albania in April 1939 and launched the rather unsuccessful Italo-Greek War in October 1940. These events resulted in Yugoslavia's geographical isolation from potential Allied support. The government tried to negotiate with the Axis on cooperation with as few concessions as possible, while attempting secret negotiations with the Allies and the Soviet Union, but these moves failed to keep the country out of the war. A secret mission to the U.S., led by the influential Serbian-Jewish Captain David Albala, with the purpose of obtaining funding to buy arms for the expected invasion went nowhere, while the Soviet Union's Joseph Stalin expelled Yugoslav ambassador Milan Gavrilović just one month after agreeing a treaty of friendship with Yugoslavia (prior to 22 June 1941, Nazi Germany and Soviet Russia adhered to the non-aggression pact the parties had signed in August 1939 and in the autumn 1940, Germany and the Soviet Union had been in talks on the USSR's potential accession to the Tripartite Pact).

1941 Under sustained German pressure and following failed attempts to maintain neutrality, Yugoslavia signed the Tripartite Pact on 25 March 1941 in the Vienna Belvedere. The following day, protests broke out in Belgrade against the accession. On 27 March 1941, Air Force general Borivoje Mirković led a group of Serbian Air Force officers in overthrowing the Cvetković government in a military coup. Hitler decided to postpone his invasion of the Soviet Union and demolish the Yugoslav state permanently.

Axis invasion and dismemberment of Yugoslavia

The April War On 6 April 1941, following Hitler's Directive No. 25 ordering the military destruction and political dismemberment of Yugoslavia, Germany, Italy, and Hungary invaded the Kingdom of Yugoslavia. Belgrade was bombed by the German Luftwaffe on the same day. The conflict, known as the April War, lasted eleven days, ending with the unconditional surrender signed by General Danilo Kalafatović on 17 April on behalf of the Yugoslav government and army. Approximately 344,000 Yugoslav officers and troops were taken prisoner, roughly half the armed forces. After most Croats and Macedonians were released, between 200,000 and 300,000, mostly Serbs, were transferred to camps in Germany and Italy. A further 300,000, again largely Serbs, escaped capture by disregarding the capitulation order. The early withdrawal of crack German units from Yugoslavia in late April, necessitated by preparations for Operation Barbarossa, prevented the Wehrmacht from fully securing the occupied territory. The Royal Yugoslav Army's arsenal fell into German hands, and King Petar II and his government fled first to Greece and subsequently to London where they established a government-in-exile.

The Yugoslav army's resistance was undermined by structural weaknesses in organisation, training, and national cohesion. Mobilisation had been deliberately withheld to avoid provoking Hitler and was not ordered until 3 April. Of roughly 700,000 men under arms, more than 400,000 had fewer than four weeks of training and not one division was at full readiness when the invasion began. The Axis brought 52 divisions: 24 German, 23 Italian, and 5 Hungarian, with 2,200 aircraft; German armoured and motorised formations that were veterans of the Polish, Norwegian and French campaigns led the assault. According to Tomasevich, the insistence of the Yugoslav army on defending all its borders assured its failure from the start.

Resistance resolve among the officer corps and general public was greater among Serbs than elsewhere in the country, with fighting resolution running highest in the period around the March 27 coup; once the bombing and invasion began defeatism spread quickly through the higher ranks of the Serbian-dominated officer corps. Elsewhere in the country resolve was present only to a limited degree, with potential fifth column risks among Ustaše sympathisers, pro-Bulgarian population in Macedonia, Albanians in Kosovo, and the Volksdeutsche minority in Vojvodina. According to Matteo J. Milazzo, German units advancing on Zagreb in the first week of fighting were received favourably by much of the Croatian civilian population. Italian military intelligence reported on 12 April 1941, five days before the Yugoslav surrender, that Croatian and Slovene reservists were abandoning their weapons and returning home, with large units falling apart. Historian Aleksa Djilas acknowledges that while some Croatian units deserted, most Croatian officers remained loyal to the Yugoslav army until 10 April when the NDH was proclaimed. Most senior commanders, convinced that resistance was hopeless, focused on surrender rather than continued defence.

Partition and occupation With Yugoslav resistance collapsed, Germany proceeded to partition the country in accordance with overlapping strategic interests: incorporating Slovene territory into the Greater Reich, exploiting Serbia's communication lines and mineral resources, rewarding Italy, Hungary and Bulgaria for their roles in the invasion and establishing an independent Croatian state as a reward for the Ustaše. Germany occupied the largest, northern part of Slovenia while Italy annexed the southern part as the Ljubljana Province and Hungary annexed the northeastern Prekmurje region. Slovene political parties had established a Slovene National Council on the first day of the invasion, but it surrendered to the Italian army on 12 April when Italian forces entered Ljubljana. Following the Yugoslav surrender, Germany occupied Serbia outright and placed it under direct military administration, the only territory in dismembered Yugoslavia under direct German military rule, administered first through a Commissioner Administration and then, from 29 August 1941, through a quisling government headed by General Milan Nedić.

On 10 April 1941, to coincide with the entry of German troops into Zagreb, Slavko Kvaternik proclaimed the Independent State of Croatia (NDH) on behalf of the Ustaše movement, with German assistance. Its territory extended over much of modern Croatia and all of modern Bosnia and Herzegovina. It functioned as an Italo-German quasi-protectorate, a puppet state, ratified under the Treaties of Rome of 18 May. The NDH was divided by a Line of Demarcation into German and Italian zones with German influence steadily growing at Italy's expense. The state owed its creation and continued existence to the two Axis powers.

Italy annexed Kosovo, coastal areas of the Croatian Littoral, large sections of Dalmatia, nearly all of the Adriatic islands, the Bay of Kotor, and gained control over Montenegro. Hungary occupied and subsequently annexed Vojvodina, Baranja, Bačka and Međimurje. The Bulgarian army moved into nearly all of modern North Macedonia and parts of eastern Serbia on 19 April 1941, with Bulgaria formally annexing these territories alongside Greek western Thrace and eastern Macedonia on 14 May. The Yugoslav government-in-exile in London retained recognition from the Allied powers.

Occupation forces