Short answers, pulled from the story.
Operation Retribution, known in German as Unternehmen Strafgericht, was the April 1941 German bombing of Belgrade ordered by Hitler in direct response to a coup d'état that had overthrown the Yugoslav government just days after it signed the Tripartite Pact. Hitler issued Directive 25 on the day of the coup, declaring that Yugoslavia must be destroyed as quickly as possible. The bombing began on the 6th of April 1941.
Casualty estimates vary widely. The occupation authorities released an official figure of 2,271 killed shortly after the attack. Other sources cited 5,000 to 10,000 fatalities, and some Yugoslav estimates went higher still. The historian Jozo Tomasevich, after reviewing postwar investigations, concluded that a figure between 3,000 and 4,000 is most realistic.
Vladimir Kren was a Royal Yugoslav Army Air Force major who flew a Potez 25 aircraft to Graz on the 3rd of April 1941 and defected to the Germans. He disclosed the locations of Yugoslavia's dispersal airfields, the air force's codes, and the positions of air-raid shelters in Belgrade, significantly weakening the city's defences before the attack. He was later arrested in Italy in March 1947, extradited to Yugoslavia, convicted of separate war crimes for his role as head of the Croatian air force, and executed in 1948.
Alexander Löhr, the commander of Luftflotte IV who organised and ordered the bombing of Belgrade, was captured by Yugoslav Partisans in May 1945, briefly escaped, and was recaptured. A Yugoslav military court tried and convicted him on war crimes charges, including his command during Operation Retribution. He was executed on the 26th of February 1947.
Yes. The National Library of Serbia was struck by bombs during the attack and gutted by fire, with hundreds of thousands of rare books, maps, and medieval manuscripts destroyed. In June 2017, it was announced that the site of the library's ruined foundations would be converted into a memorial garden.
No. 37 Squadron of the Royal Air Force carried out two bombing raids on Sofia, the Bulgarian capital, on the nights of 6/the 7th of April and 12/the 13th of April 1941, dropping a total of 30 tonnes of high-explosive bombs on railway targets and nearby residential areas. Britain was not formally at war with Bulgaria until the 12th of December 1941, and the historian Herman Knell described the retaliatory justification for these raids as "strange and implausible".