Invasion of Yugoslavia

The invasion of Yugoslavia, also known as the April War or Operation 25, was a German-led attack on the Kingdom of Yugoslavia by the Axis powers which began on 6 April 1941 during World War II. The order for the invasion was put forward in "Führer Directive No. 25", which Adolf Hitler issued on 27 March 1941, following a Yugoslav coup d'état that overthrew the pro-Axis government.

The invasion commenced with an overwhelming air attack on Belgrade and facilities of the Royal Yugoslav Air Force (VVKJ) by the Luftwaffe (German Air Force) and attacks by German land forces from southwestern Bulgaria. These attacks were followed by German thrusts from Romania, Hungary and the Ostmark (modern-day Austria, then part of Germany). Italian forces were limited to air and artillery attacks until 11 April, when the Italian Army attacked towards Ljubljana (in modern-day Slovenia) and through Istria and Lika and down the Dalmatian coast. On the same day, Hungarian forces entered Yugoslav Bačka and Baranya, but like the Italians they faced practically light resistance. Italians moved into Dalmatia also from Italian-controlled Albania, after repelling an initial Yugoslav attack there.

Scholars have proposed several theories to explain the sudden collapse of the Royal Yugoslav forces, including poor training and equipment, generals eager to secure a quick cessation of hostilities, and fifth column activities by groups of Croatians, Slovenians, and ethnic Germans. The latter has been questioned by scholars who have suggested that the fifth column had little effect on the ultimate outcome. The invasion ended when an armistice was signed on 17 April 1941, based on the unconditional surrender of the Yugoslav army, which came into effect at noon on 18 April. The Kingdom of Yugoslavia was then occupied and partitioned by the Axis powers. Most of Serbia and the Banat became a German zone of occupation while other areas of Yugoslavia were annexed by neighboring Axis countries, Germany, Hungary, Italy, Albania and Bulgaria. Croatia became the Independent State of Croatia (NDH), an Axis puppet state created during the invasion comprising the Srem, Bosnia and Herzegovina as well as the Croatian lands. Along with Italy's stalled invasion of Greece on 28 October 1940, and the German-led invasion of Greece (Operation Marita) and invasion of Crete (Operation Merkur), the invasion of Yugoslavia was part of the German Balkan Campaign (German: Balkanfeldzug).

In October 1940, Fascist Italy had attacked the Kingdom of Greece only to be forced back into Italian-occupied Albania. German dictator Adolf Hitler recognised the need to go to the aid of his ally, Italian dictator Benito Mussolini. Hitler did this not only to restore diminished Axis prestige, but also to prevent Britain from bombing the Romanian Ploiești oilfields from which Nazi Germany obtained most of its oil.

In 1940 and early 1941, Hungary, Romania, and Bulgaria all agreed to adhere to the Tripartite Pact and thus join the Axis. Hitler then pressured Yugoslavia to join as well. The Regent, Prince Paul, yielded to this pressure, and declared Yugoslavia's accession to the Pact on 25 March 1941. This move was highly unpopular with the Serb-dominated officer corps of the military, Serbian organizations such as National Defense and the Chetniks Association, the Serbian Orthodox Church, a large part of the Serbian population as well as liberals and Communists. Military officers (predominantly Serbs) executed a coup d'état on 27 March 1941, forced the Regent to resign, and declared 17-year-old King Peter II to be of age.

Upon hearing news of the coup in Yugoslavia, Hitler called his military advisers to Berlin on 27 March. On the same day as the coup he issued Führer Directive No. 25, which called for Yugoslavia to be treated as a hostile state. Hitler took the coup as a personal insult, and was so angered that he was determined, in his words, "to destroy Yugoslavia militarily and as a state" (Jugoslawien militärisch und als Staatsgebilde zu zerschlagen) and to do so "with pitiless harshness" and "without waiting for possible declarations of loyalty of the new government".

Hungary had joined the Tripartite Pact on 20 November 1940. On 12 December, it also concluded a treaty with Yugoslavia calling for "permanent peace and eternal friendship". The Hungarian leadership was split after Germany's War Directive 25 was delivered on 27 March 1941. Regent Miklós Horthy and the military favoured taking part in the invasion of Yugoslavia and mobilized the following day. Prime Minister Pál Teleki sought to prevent German troops passing through Hungary and cited the peace treaty with Yugoslavia as an impediment to co-operation with the Germans.

On 1 April, Yugoslavia redesignated its Assault Command as the Chetnik Command, named after the Serb guerrilla forces from World War I, which had resisted the Central Powers. The command was intended to lead a guerrilla war if the country was occupied. Its headquarters was transferred from Novi Sad to Kraljevo in south-central Serbia on 1 April.