The German air force launches Operation Castigo, the bombing of Belgrade, on April 6, 1941, as 24 divisions and 1,200 tanks drive into Greece. The attack on Yugoslavia was swift and brutal, an act of terror resulting in the death of 17,000 civilians—the largest number of civilian casualties in a single day since the start of the war. Making the slaughter all the worse was that nearby towns and villages had emptied out into the capital city to celebrate Palm Sunday. All of Yugoslavia’s airfields were also bombed, destroying most of its 600 aircraft while still on the ground. As part of a comprehensive Balkan offensive, German forces also bombed the Greek port city of Piraeus as army divisions swept south and west, en route to Salonica and the eventual occupation of Greece. Also on this day: British General Alan Cunningham’s troops enter Ethiopia’s capital, Addis Ababa, formally expelling the Italian occupiers and setting the stage for the return of Ethiopia’s emperor, Haile Selassie.