Once the decision was made to attack Yugoslavia as well as Greece, plans were altered to pull in some forces intended for Barbarossa. Luftflotte 4, in Austria, was tasked to operate over Yugoslavia. The German 2nd Army would attack from Austria and Hungary. The 12th Army, in Bulgaria, would still attack Greece. The German airforces involved dwarfed those available to the British. Luftflotte 4 had 576 aircraft, alone. Another 168 were pulled in from Fliegerkorps X. The air support for the 12th Army was provided by Fliegerkorps VIII, which had 414 aircraft.
The situation in Yugoslavia was such that the country was doomed in a couple of days. When the attack commenced on 6 April 1941, the 9th Panzer Division, of the 40th Corps was a at Skopje. The 73rd Infantry Division moved to Prilep. Armoured forces reached Salonika on 9 April.
In air attacks on the Piraeus on the night of 6 and 7 April, the Clan Fraser was bombed, caught fire, and exploded. The explosion of the Clan Fraser's cargo of 250 tons of explosives sank other ships and damaged the dock facilities. The damage was so extensive as to shut down the port.
On that night of 6 and 7 April, Sofia was bombed by 6 Wellingtons, hitting an ammunition train and damaging the railway in Sofia. Blenheims from No.84 Squadron bombed a railway station 50 miles south of Sofia. This is based on the account in Vol.II of the Official History.