1941 Axis campaign during World War II
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. A Yugoslav attack into the northern parts of Italian-controlled Albania met with initial success, but was ultimately repelled into Dalmatia. 
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 (, or 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 ().
 ;templatestyles src="Template:TOC limit/styles.css" / ;
Background.
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.
Preparation.
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 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.
On 2 April, the German ambassador having already been recalled for "talks", the remaining embassy staff were ordered to leave the capital and to warn the embassies of friendly nations to likewise evacuate. That sent the unmistakable message that Yugoslavia was about to be invaded.
On 3 April, Hitler issued War Directive 26 detailing the plan of attack and command structure for the invasion as well as promising Hungary territorial gains. The same day Teleki killed himself. Horthy, seeking a compromise, informed Hitler that evening that Hungary would abide by the treaty, though it would likely cease to apply should Croatia secede, and Yugoslavia cease to exist. Upon the proclamation of an Independent State of Croatia in Zagreb on 10 April, this scenario was realized, and Hungary joined the invasion, its army crossing into Yugoslavia the following day.
Opposing forces.
Axis order of battle.
The invasion was spearheaded by the German 2nd Army with elements of the 12th Army, First Panzer Group, and an independent panzer corps combined with overwhelming "Luftwaffe" support. The 19 German divisions included five panzer divisions, two motorised infantry divisions and two mountain divisions. The German force also included three well-equipped independent motorised infantry regiments and was supported by over 750 aircraft. The Italian 2nd Army and 9th Army committed a total of 22 divisions and 666 aircraft to the operation. The Hungarian 3rd Army also participated in the invasion, with support available from over 500 aircraft.
During the April War, the "Führer" Headquarters (FHQ) was codenamed "" (Spring Storm) and consisted of the "Führersonderzug" (Special "Führer"'s Train) codenamed "Amerika" stationed in Mönichkirchen alongside the special train "Atlas" of the Armed Forces Operations Staff ("Wehrmachtführungsstabes", WFSt). "Atlas" did not arrive at Mönichkirchen until 11 April, well after operations were underway, and "Amerika" only arrived the following day. Mönichkirchen was chosen because a nearby rail tunnel could provide shelter in the event of air attack. Both trains returned to Berlin on 26 April.
After the Italian invasion in the northwest began, King Victor Emmanuel III moved to a villa owned by the Pirzio Biroli family at Brazzacco, near Moruzzo, in order to be close to the front.
Germany attacked Yugoslavia from bases in three countries besides itself: Hungary, Romania, and Bulgaria. German troops entered each of these countries under different pretenses and at different times. The first country to receive a German military mission was Romania. Ostensibly to train the Romanian armed forces, its real purpose was to protect Romania's petroleum resources and prepare for an attack on the Soviet Union. The "Wehrmacht" entered Bulgaria more circumspectly, first with the intention of providing aerial defense against any force attacking Romania's oilfields and later with that of invading Greece in support of Italy. German troops did not enter Hungary until the attack on Yugoslavia was already planned and Hungary's participation had been secured.
Deployment in Romania.
King Carol II of Romania, starting from the cession of Bessarabia and Northern Bukovina to the Soviet Union, proposed in a letter to Adolf Hitler on 2 July 1940 that Germany send a military mission to Romania. The Romanian government asked that a mission be sent urgently on 7 September 1940, the day after Carol's abdication. The decision to aid Romania was taken on 19 September, and Hungary was asked to provide transit to German soldiers on 30 September. The first troops entered Romania on 10 October. They entered Bucharest two days later (12 October) to shouts of "Heil!" The official explanation for the presence of German troops was that they were there to train the Romanian army. Hitler's directive to the troops on 10 October had stated that "it is necessary to avoid even the slightest semblance of military occupation of Romania." In the second half of October, the Romanian leader, Ion Antonescu, asked that the military mission be expanded. The Germans happily obliged the request, since the oil fields and refineries at Ploiești were vital to their war effort. Romania was also an important launching point for an attack on the Soviet Union, which made the presence of German troops a violation of the Molotov–Ribbentrop Pact of 23 August 1939.
By the middle of November, the 13th Motorised Infantry Division had been assembled in Romania and reinforced by the 4th Panzer Regiment, engineers and signal troops, as well as six fighter and two reconnaissance "Luftwaffe" squadrons, and some antiaircraft artillery. A total of seventy batteries of artillery were moved into Romania. On 23 November, Romania signed the Tripartite Pact. At the time Germany informed Romania that she would not be expected to participate in an attack on Greece, but that Germany wanted to use Romanian territory to provide a base for a German attack. On 24 November, Antonescu met with Wilhelm Keitel, chief of the "Oberkommando der Wehrmacht", to discuss common defense. As a result of this meeting, the 16th Panzer Division was sent to Romania in late December. The 12th Army and First Panzer Group, along with heavy bridging equipment for the planned crossing of the Danube, followed in January 1941. By January 1941 the total number of German effectives in Romania was 170,639. Those elements of the 12th Army that were to invade Yugoslavia from Romania assembled near Timișoara (Temeschwar).
Between November 1940 and February 1941, the "Luftwaffe" gradually moved 135 fighters and reconnaissance aircraft into Romania (in 22–26 squadrons). In early April 1941 they moved a further 600 aircraft from France, Africa, and Sicily into Romania and Bulgaria in a period of ten days. The fighter and reconnaissance craft were sent to fields in Arad, Deva, and Turnu Severin. On 12 February Britain broke off diplomatic relations with Romania on the grounds that it was an enemy-occupied country.
Deployment in Bulgaria.
Two events in early November 1940 convinced Hitler of the need to station troops, especially the "Luftwaffe", in Bulgaria. The first was false reports that the British were constructing an airfield on Lemnos, from which they could bomb Ploiești. The second was the beginning of British air raids originating from Greek bases against Italian shipping on 6 November. Planning for the German invasion of Greece from Bulgaria began on 12 November.
Already on 13 November, the Soviets were (incorrectly) accusing the Germans of having troops in neutral Bulgaria. On 18 November, Tsar Boris III of Bulgaria met with Hitler and promised to participate in an attack on Greece, but only at the last moment. Shortly thereafter a secret German team under Colonel Kurt Zeitzler entered Bulgaria to establish fuel depots, arrange for troop billeting and scout the terrain. They were soon followed by hundreds of "Luftwaffe" personnel to establish air observation stations. By the end of December over a thousand German troops in civilian clothing were active in Bulgaria, although the latter's government continued to deny it. Bombers and dive-bombers were also gradually moved into Bulgaria, beginning in November. By the end of March 1941, the "Luftwaffe" had 355 aircraft in the country.
On 17 February 1941, Bulgaria signed a non-aggression pact with Turkey, paving the way for its adherence to the Tripartite Pact, which was signed by Prime Minister Bogdan Filov in Vienna on 1 March. When Ivan V. Petrov, member of the National Assembly from Yablanitsa, asked why the Assembly had not been consulted, Filov pointed out that the constitution only required parliamentary approval prior to ratification. The signing was ratified by a vote in the Assembly of 140 to 20. The first German troops crossed