'Castigo' was the German 'terror bombing' of Belgrade, the Yugoslav capital, within the context of 'Unternehmen 25' and otherwise known as 'Bestrafung' and 'Strafgericht' (6/8 April 1941).

The operation paralysed the Yugoslav political, civilian and military command and control structures, inflicted widespread destruction largely in the centre of the city, and caused large numbers of civilian casualties. The 'Castigo' bombing of Belgrade was combined with air attacks on many of the Yugoslav air force airfields and other strategic targets across Yugoslavia, and contributed decisively to the rapid collapse of Yugoslav resistance.

After the union of Austria with Germany in 1938, Germany now shared a border with Yugoslavia in the latter’s north-western area, and steadily increased its pressure on Yugoslavia to join the alliance between Germany and Italy, which was expanded into the Axis alliance by the Tripartite Pact between Germany, Italy and Japan on 27 September 1940. Further pressure on Yugoslavia along these lines had begun in April 1939, when Yugoslavia gained a second frontier with a forthcoming Axis state, this time in its south-western area, when Italy invaded Albania. The almost total encirclement of Yugoslavia by the alliance dominated by Germany was completed when other Yugoslav neighbours, in the form of Hungary, Romania and Bulgaria joined the Tripartite Pact on 20 November 1940, 23 November 1940 and 1 March 1941 respectively. Thus the only part of Yugoslavia not contiguous with an Axis state was its extreme southern border with Greece, which had been invaded by Italy in 'Emergenza G' on 28 October 1940.

It thus became increasingly difficult for Yugoslavia to maintain its neutral stance, and on 14 February 1941 Adolf Hitler invited the Yugoslav prime minister, Dragisa Cvetković, and his foreign minister, Aleksandar Cincar-Marković, to Berchtesgaden, where he requested that Yugoslavia also join the Tripartite Pact, just two weeks before Bulgaria commitment to the pact. On the following day, German troops entered Bulgaria from Romania.

Hitler applied more pressure on 4 March, when Prince Pavle, the regent for King Petar II, who was only 17 years old, visited Berchtesgaden, but the prince delayed a decision. On 7 March, British troops began landing in Greece in 'Lustre' to bolster the defences of their Balkan ally against the Italians. Desiring to secure the southern flank of his impending 'Barbarossa' invasion of the USSR, Hitler now demanded that Yugoslavia sign the pact, and the Yugoslav government eventually complied on 25 March. Two days later a military coup d'état was carried out by a group of air force and royal guard officers led by Brigadier General Borivoje Mirković. Pavle was deposed and replaced by Petar II, who was then declared to be of age.

On the day of this Yugoslav coup, Hitler issued his Führerweisung Nr 25, which stated that the event had changed the political situation in the Balkans. He ordered that 'even if Yugoslavia at first provides declarations of loyalty, she must be considered as a foe and therefore must be destroyed as quickly as possible'.

Hitler decided that Belgrade would be bombed in 'retribution' for the coup against the government which had only just signed the pact. In order to carry out Hitler’s order, on 27 and 28 March 1941 Reichsmarschall Hermann Göring transferred about 500 fighter and bomber aircraft from France and northern Germany. The commander of Luftflotte IV, Generaloberst Alexander Löhr, organised the attack on the Yugoslav capital by these warplanes operating in waves by day and night, and issued his orders for the bombing on 31 March, though the decision was not confirmed by Hitler until 5 April. Although Hitler ordered the general destruction of Belgrade, Löhr replaced these general directions with specific military objectives at the last minute.

On 6 April 1941, as the 'Unternehmen 25' land invasion began, a massive air offensive was unleashed on Belgrade and every airfield and installation of the Yugoslav air force without any declaration of war. Within this context bombers and dive-bombers dropped between 215 and 360 tons of bombs and incendiaries on the Yugoslav capital in 'Castigo'.

The Luftwaffe units involved were elements of General Wolfram Freiherr von Richthofen’s Fliegerkorps VIII, and these included the level bombers of Oberst Herbert Riekhoff’s Kampfgeschwader 2, Obert Hans-Joachim Rath’s KG 4 and Major Hans Bruno Schulz-Heyn’s KG 51, and the dive-bombers of Major Graf Clemens von Schönborn-Wiesentheid’s Sturzkampfgeschwader 77, with protection provided by the fighters of Oberstleutnant Hannes Trautloft’s Jagdgeschwader 54.

The first wave of the assault, consisting of between 150 and 234 bombers and dive-bombers escorted by 120 fighters, fell on Belgrade at 07.00. In response the Yugoslav air force scrambled 20 Messerschmitt Bf 109, 18 Hawker Hurricane and six locally manufactured Rogozarski IK-3 single-engined fighters. Some of the Hurricane pilots engaged friendly Messerschmitt Bf 109 single-engined fighters, mistaking them for German aircraft, as well as attacking Luftwaffe aircraft. About 50 Yugoslav aircraft were destroyed by the first wave of strikes. Later in the same day, three more waves of German aircraft, each of about 100 machines, attacked Belgrade.

The bombing continued along the same basic line on 7 April. Despite its weakness, the Yugoslav air force and the inadequate anti-aircraft defences of Belgrade tried to meet the overwhelming Luftwaffe assault, but were effectively destroyed during the first wave of the attack. There are different accounts of the degree of success achieved by the defenders: a US study of 1953 stated that the Luftwaffe lost two fighters, and in exchange shot down 20 Yugoslav aircraft and destroyed 44 more on the ground, while another source states that the Yugoslavs shot down 40 German aircraft in the course of the first two days of the air battle.

Whatever their successes, the Yugoslav defences were soon so shattered that the German dive-bombers were able to operate at roof-top altitude in later attacks.

The bombing of Belgrade lasted for three days or, according to most sources, just two days as a result of poor flying weather on 8 April.

The most important Yugoslav cultural institution to be destroyed was the National Library of Serbia, which was hit by bombs and then gutted by fire. Some 300,000 volumes, rare books, maps and mediaeval manuscripts were destroyed.

No. 37 Squadron of the Royal Air Force undertook two bombing raids on Sofia, the capital of Bulgaria, in retaliation for the bombing of Belgrade. Flying Vickers Wellington twin-engined medium bombers from a Greek airfield, the squadron flew these raids on 6/7 April and 12/13 April, dropping a total of 30 tons of bombs on railway targets and nearby residential areas. These raids were carried out despite the fact that the UK was not formally at war with Bulgaria until 12 December 1941.

The Yugoslav civilian casualties were significant, but figures between 1,500 and 17,000 dead have been reported, with one source averring that some 24,000 corpses were recovered from the ruins, and those of many other dead were never found. The official casualty figure soon after the bombing was 2,271 killed.

After the surrender of Yugoslavia, Luftwaffe technical staff undertook a bomb damage assessment in Belgrade. The report stated that 218.5 tonnes of bombs were dropped, with some 10 to 14% of this tonnage represented by incendiaries. The report listed all the targets of the bombing, which included the royal palace, the war ministry, military headquarters, the central post office, the telegraph office, passenger and goods railway stations, power stations and barracks. It also mentioned that seven aerial mines were dropped, and that areas in the centre and northwest of the city had been destroyed, comprising some 20 to 25% of its total area. Some aspects of the bombing remain unexplained, particularly the use of the aerial mines.

Another account, however, states that almost 50% of housing in Belgrade was destroyed. After the invasion, the Germans forced 3,500 to 4,000 Jews to collect rubble.