the royal yugoslav army air force ( unk ) had only 77 modern fighter aircraft available to defend Defending belgrade against the hundreds of german fighters and bombers that struck Attack in the first wave early on 6 april .

the invasion Attack commenced Process_start with an overwhelming air attack Attack on belgrade and facilities of the royal yugoslav air force ( unk ) by the luftwaffe ( german air force ) and attacks Attack by german land forces from southwestern bulgaria .