German bombing
At the end of March 1941 the Kingdom of Yugoslavia was forced to join the Axis, which ignited a popular revolution in the country; this led to a coup d'etat and a switch in foreign policy with a re-orientation to join the Allies. This happened in a crucial moment, when the Wehrmacht was in the final stages of preparation for Operation Barbarossa against the Soviet Union. The successful opening of a campaign in the east required either a diplomatically secured or militarily subdued Balkans - free from the possibility of British intervention - and it was this act of Yugoslavian defiance which resulted in Hitler's order for immediate punitive action.
Operation Punishment was the code name for the German bombing of Belgrade during the invasion of Yugoslavia. The Luftwaffe bombed the city on April 6 (Palm Sunday) without a declaration of war, continuing bombing until April 10. More than 500 bombing sorties were flown against Belgrade in three waves coming from Romania where German forces were assembled for the attack on the Soviet Union. Most of the government officials fled, and the Yugoslav army began to collapse.
The attack on Yugoslavia was one of the earliest terror bombings of World War II. In following days Bulgaria, Hungary, and Italy joined Nazi Germany in partitioning Kingdom of Yugoslavia, with the support of the newly established Nazi-puppet Independent State of Croatia. The country was absorbed within 12 days, and Greece fell a week later, which made the Third Reich the master of most of continental Europe and ready for launching the attack against Soviet Union.
Waves of Luftwaffe heavy bombers and Stuka dive bombers bombed Belgrade, killing an estimated 2,274 civilians during the initial April 6-7 bombardment - though some estimates put the figure as high as 5,000 to 17,000 Serbian civilians killed - and several hundreds of buildings were destroyed. The most important cultural institution that was destroyed was the National Library of Serbia, with 300,000 books, including priceless Medieval manuscripts.
German Field Marshal von Kleist said during his trial after the war: "The air raid on Belgrade in 1941 had a primarily political-terrorist character and had nothing to do with the war. That air bombing was a matter of Hitler's vanity, his personal revenge." The bombing without a declaration of war become one of the prosecution's charges which led to the execution of the commander of the Luftwaffe formations involved, General Löhr.
The last stand of the Kingdom of Yugoslavia
The bombing of Belgrade on April 6 was the last stand the Kingdom took against the Axis powers. Although information is mostly sketchy, many sources say that Axis countries (in this case, Germany, Italy, and probably Hungary) deployed up to 2,000 combat aircraft during the bombing of Belgrade. The Royal Yugoslav Air Force's defenses were mostly obsolete. Many of its fighter planes were obsolete, some even with World War I-era technology. Out of 487 operational combat aircraft in the inventory of the Yugoslav Air Force in 1941, less than 150 of them were fit enough to pose a threat to modern Nazi fighters. During April 6th and 7th of 1941, the Yugoslav Air Force only downed about 90 to 100 enemy aircraft. [1]
When the bombing began on the morning of April 6, 1941, many flak defenses near Belgrade were destroyed by Nazi bombers. Even though the strength of the German bomber force was great enough to kill several thousand Belgrade civilians, it took as long as several hours for many of the Yugoslav Air Force squadrons to find out that the country was actually being bombed. However, their finest hours were to be met. On April 6, a group of 18 Yugoslav fighters (6 Icarus IK-3s, and ironically, 12 German-made Messerschmitt Bf 109s) downed 12 German Messerschmitts. [2]
Milisav Semiz was the leading Yugoslav ace of the war, downing four German planes before himself was shot down in his IK-3.
.After poland was defeated what ever happened to them didn't alter the course of the war .If the soviet system had been diffrent maybe not so many of them would have dieded.
I am saying the following, however: when it happened [the attack on USSR, against all the best "intelligence" service's advice Stalin was given - he did nothing or the opposite to the advice given!] a massive effort was mounted to remove the industry deeper into the USSR and a lot of suffering ensued, because of his idiotic emotional strongman-to-strongman "we understand each other" and have an agreement twatism.
puts a different perspective on things than the one in the media!! 
Much readier to accept the yoke/"authority" - one has to use much more force in the East to get the same results/power!!!