The Combined Bomber Offensive (CBO) was an Anglo-American offensive of strategic bombing during World War II in Europe. The primary portion of the CBO was against Luftwaffe targets which was the highest priority from June 1943 to 1 April 1944. The subsequent highest priority campaigns were against V-weapon installations (June 1944) and petroleum, oil, and lubrication (POL) plants (September 1944). Additional CBO targets included railyards and other transportation targets, particularly prior to the invasion of Normandy and, along with army equipment, in the final stages of the War in Europe.
The British bombing campaign was chiefly waged by night by large numbers of heavy bombers until the latter stages of the war when German fighter defences were so reduced that daylight bombing was possible without risking large losses. The US effort was by day – massed formations of bombers with escorting fighters. Together they made up a round-the-clock bombing effort except where weather conditions prevented operations.