The ground invasion had begun a few hours earlier, and air attacks were also made on VVKJ airfields and other strategic targets across Yugoslavia.

Among the non-military targets struck during the bombing were the National Library of Serbia, which burned to the ground with the loss of hundreds of thousands of books and manuscripts, and the Belgrade Zoo.

Kren was arrested in 1947 on unrelated charges of war crimes stemming from his subsequent service as the head of the Air Force of the Independent State of Croatia.

Following Germany's 1938 Anschluss of Austria, Yugoslavia shared a border with the Third Reich and came under increasing pro-Axis political pressure as its neighbours fell into line with the Axis powers.

[4] From that time, Yugoslavia was almost surrounded by Axis powers or their client states, and its neutral stance toward the war was under tremendous pressure.

[6] On 6 March, the Royal Yugoslav Army Air Force (Serbo-Croatian: Vazduhoplovstvo Vojske Kraljevine Jugoslavije, VVKJ) was secretly mobilised.

[7] Hitler, wishing to secure his southern flank in anticipation of Germany's impending invasion of the Soviet Union, demanded that Yugoslavia sign the Pact.

[9] Two days later, a group of VVKJ and Royal Guard officers, led by Brigadier General Borivoje Mirković, deposed Prince Paul in a coup d'état.

The German incursions demonstrated that the Yugoslav ground observation post network and supporting radio communications were inadequate.

[7] Hitler decided that Belgrade would be bombed as punishment for the coup, under the codename Operation Retribution (Unternehmen Strafgericht).

On 27 and 28 March 1941, Reichsmarschall Hermann Göring transferred about 500 fighter and bomber aircraft from France and northern Germany to airfields near the Yugoslav border.

The commander of Luftflotte IV, Generaloberst (General) Alexander Löhr, allocated these aircraft to attack the Yugoslav capital in waves by day and night.

[15] On the afternoon of 5 April, a British colonel visited Mirković at the VVKJ base in Zemun and informed him that the attack on Belgrade would commence at 06:30 the next day.

Other than a small number of locally made Rogožarski IK-3 fighters, almost all the modern aircraft available to the VVKJ were of German, Italian or British design for which limited spares and munitions were available.

[17] German ground forces crossed the Yugoslav border at 05:15 on 6 April, and the Reich Minister of Propaganda, Joseph Goebbels, announced Germany's declaration of war at 06:00.

[18] Yugoslav anti-aircraft defences caused a false alarm when they reported the approach of an air raid from the direction of Romania at 03:00, but listening posts on the Romanian border had heard the aircraft engines of the Romanian-based Fliegerführer Arad warming up well before they took off.

[23] The first wave hit the Belgrade power station, the post office including telegraph and postal services, the headquarters of the Ministry of Army and Navy, the Yugoslav Supreme Command building, the military academy, the royal palace at Dedinje, the royal guard barracks at Topčider, the gendarmerie command headquarters, and the airport at Zemun, among other targets.

[27][28] The weak VVKJ and inadequate anti-aircraft defences of Belgrade briefly attempted to meet the overwhelming Luftwaffe assault, but were eliminated as threats during the first wave of the attack.

[25] Other sources state the air battle over Belgrade lasted only two days owing to poor flying conditions on 8 April.

[36][37] The historian Jozo Tomasevich writes that the higher estimates were downgraded following "careful postwar investigations", and indicates that a figure between 3,000 and 4,000 is more realistic.

He was intensively interrogated, after which he was tried before a Yugoslav military court on war crimes charges, one of which related to his command of Luftflotte IV during Operation Retribution.

[44] Following the invasion, Kren was appointed head of the Air Force of the Independent State of Croatia (Croatian: Zrakoplovstvo Nezavisne Države Hrvatske, ZNDH).

He was arrested in Italy in March 1947 and extradited to Yugoslavia, where he was tried on unrelated charges of war crimes for his role in the targeting of civilians by the ZNDH.

[47] The Serbian American poet Charles Simic, a survivor of the bombing, wrote a poem titled "Cameo Appearance" recounting his experiences.

[49] On 6 April 2016, the 75th anniversary of the bombing, a memorial service was held for the victims, attended by the Serbian Minister for Labour, Employment, Veteran and Social Policy, Aleksandar Vulin.