The Wikipedia article of the day for April 6, 2020 is
Operation Retribution (1941).
Operation Retribution was the April 1941 German bombing of Belgrade, the capital of the Kingdom of Yugoslavia, in retaliation for the coup d'état that overthrew the government that had signed the Tripartite Pact. The bombing occurred in the first days of the invasion of Yugoslavia by German-led Axis forces during World War II. The Royal Yugoslav Army Air Force had only 77 modern fighter aircraft available to defend Belgrade against the hundreds of German fighters and bombers that struck in the first wave early on 6 April. Three days prior, Major Vladimir Kren had defected to the Germans, elucidating the air force's codes and disclosing the locations of military assets. Three more waves of bombers attacked Belgrade on 6 April, and more attacks followed in subsequent days. The attacks resulted in the paralysis of Yugoslav civilian and military command and control, the widespread destruction of Belgrade's infrastructure, and many civilian casualties.
