ual ties . ▁The ▁ground ▁invasion ▁had ▁begun ▁a ▁few ▁hours ▁earlier , ▁and ▁air ▁attacks ▁were ▁also ▁made ▁on ▁V V K J ▁air fields ▁and ▁other ▁strateg ic ▁targets ▁across ▁Y ug oslav ia . ▁Among ▁the ▁non - mil it ary ▁targets ▁struck ▁during ▁the ▁bomb ing ▁were ▁the ▁National ▁Library ▁of ▁Ser bia , ▁which ▁burn ed ▁to ▁the ▁ground ▁with ▁the ▁loss ▁of ▁hundreds ▁of ▁thousands ▁of ▁books ▁and ▁manuscript s , ▁and ▁the ▁Bel grade ▁Z oo . ▁ ▁The ▁Royal ▁Air ▁Force ▁carried ▁out ▁two ▁bomb ing ▁ra ids ▁on ▁Sof ia , ▁the ▁capital ▁of ▁Bulg aria , ▁in ▁ret ali ation ▁for ▁the ▁attacks ▁on ▁Y ug oslav ia , ▁which ▁surrender ed ▁on ▁ 1 7 ▁April . ▁The ▁senior ▁Luft wa ffe ▁officer ▁responsible ▁for ▁the ▁bomb ing , ▁General ober st ▁Alexander ▁Lö hr , ▁was ▁captured ▁by ▁the ▁Y ug oslav s ▁at ▁the ▁end ▁of ▁the ▁war ▁and ▁was ▁tried ▁and ▁executed ▁for ▁war ▁cr imes , ▁in ▁part ▁for ▁his ▁invol vement ▁in ▁the ▁bomb ing ▁of ▁Bel grade . ▁K ren ▁was ▁arrested ▁in ▁ 1 9 4 7 ▁on ▁un related ▁charges ▁of ▁war ▁cr imes ▁stem ming ▁from ▁his ▁subsequent ▁service ▁as ▁the ▁head ▁of ▁the ▁Air ▁Force ▁of ▁the ▁Independent ▁State ▁of ▁Cro atia . ▁He ▁was ▁extr ad ited ▁to ▁Y ug oslav ia ▁to ▁face ▁trial , ▁conv icted ▁on ▁all ▁counts , ▁and ▁executed ▁in ▁ 1 9 4 8 .