Short answers, pulled from the story.
The first wave of the German bombing of Belgrade began at 06:30 on the 6th of April 1941. This attack followed a coup d'état that occurred two days after Yugoslavia signed the Tripartite Pact on the 25th of March 1941.
Historians estimate between 3,000 and 4,000 fatalities from the German bombing of Belgrade. The official figure released by occupation authorities shortly after the event was 2,271 killed, while sources vary with estimates ranging up to 17,000.
General Alexander Löhr was convicted of war crimes related to his command of Luftflotte IV during Operation Retribution and executed on the 26th of February 1947. He had been captured by Yugoslav Partisans on the 9th of May 1945 before escaping and being recaptured on the 13th of May.
The National Library of Serbia burned to the ground during the German bombing of Belgrade, resulting in the loss of hundreds of thousands of books. The ruins of this site were announced in June 2017 to become a memorial garden.
Major Vladimir Kren flew a Potez 25 aircraft to Graz on the 3rd of April and defected to the Germans. He disclosed locations of dispersal airfields and codes used by the Royal Yugoslav Army Air Force, which included troop mobilization centers and air-raid shelter positions within Belgrade.