Ploesti oil fields in Romania were main source of oil for Nazi Germany. Therefore they were considered key target for Allied bombers. However, since they were far away from bases in Britain, it was not easy target. After minor attack in June 1942, and massive attack of 178 B-24 bombers on August 1st 1944, casualties were high, and effectiveness of of bombing was limited.
After Allies created bases near Foggia in Italy, it was possible to make constant successful attacks. During them, hundreds of planes were shot down. Many were damaged, and those crews who could not reach bases in Italy, tried to Evade in Yugoslavia with help of Partisan and Chetnik Guerrilla movements. Almost 3/4 of Airmen rescued during Halyard mission were shot down on missions to Ploesti, or line of oil supply in Romania. That clearly shows importance of Ploesti in Allied planning. On photo, US B-24D bombers flying over burning Ploesti oil facilities.