, ▁and ▁six ▁Rog o ž ar ski ▁I K - 3 s . ▁ ▁Bomb ing ▁▁ ▁German ▁ground ▁forces ▁crossed ▁the ▁Y ug oslav ▁border ▁at ▁ 0 5 : 1 5 ▁on ▁ 6 ▁April , ▁and ▁the ▁Reich ▁Minister ▁of ▁Pro pag anda , ▁Joseph ▁Go ebb els , ▁announced ▁Germany ' s ▁declaration ▁of ▁war ▁at ▁ 0 6 : 0 0 . ▁The ▁invasion ▁and ▁concurrent ▁bomb ing ▁coinc ided ▁with ▁E aster ▁Sunday ▁as ▁observed ▁by ▁the ▁Ser bian ▁Orth odox ▁Church , ▁which ▁uses ▁the ▁Julian ▁calendar . ▁Y ug oslav ▁anti - air craft ▁def ences ▁caused ▁a ▁false ▁alarm ▁when ▁they ▁reported ▁the ▁approach ▁of ▁an ▁air ▁ra id ▁from ▁the ▁direction ▁of ▁Roman ia ▁at ▁ 0 3 : 0 0 , ▁but ▁listening ▁posts ▁on ▁the ▁Roman ian ▁border ▁had ▁heard ▁the ▁aircraft ▁engines ▁of ▁the ▁Roman ian - based ▁Fl ie ger führ er ▁Ar ad ▁war ming ▁up ▁well ▁before ▁they ▁took ▁off . ▁The ▁V V K J ' s ▁ 5 1 st ▁F ighter ▁Group ▁at ▁Z em un ▁had ▁been ▁alert ed ▁before ▁dawn , ▁and ▁when ▁reports ▁began ▁to ▁be ▁received ▁about ▁Luft wa ffe ▁attacks ▁on ▁V V K J ▁air fields , ▁the ▁first ▁pat rol ▁was ▁sent ▁into ▁the ▁air . ▁At ▁first , ▁no ▁aircraft ▁could ▁be ▁seen ▁approaching ▁Bel grade . ▁ ▁The ▁first ▁wave ▁closed ▁on ▁Bel grade ▁between ▁ 0 6 : 3 0 ▁and ▁ 0