On 15 September 1940, during the Second World War, an air battle was fought during the Battle of Britain between the German Luftwaffe and the Royal Air Force (RAF). Now celebrated as Battle of Britain Day, on that date, the Luftwaffe launched its largest and most concentrated attack against London in order to force the RAF into a battle of annihilation. The German objective was to gain air superiority over the English Channel and allow the invasion of England, Operation Sea Lion, to proceed.
During the course of the day, around 1,500 aircraft took part in the air battles. The action was the climax of the Battle of Britain. The RAF's Fighter Command defeated the German raids; the Luftwaffe formations were dispersed by a large cloud base and failed to inflict severe damage on the city of London. In the aftermath of the raid, Operation Sea Lion was postponed. Having been defeated in daylight, the Luftwaffe turned its attention to the Blitz night campaign, which lasted until May 1941.
Background
By June 1940, Nazi Germany had conquered most of Western Europe and much of Scandinavia. At that time, the only major power standing in the way of a German-dominated Europe was Great Britain. In the absence of offers of peace from the British, Adolf Hitler ordered the Luftwaffe (German Air Force) to destroy the RAF in order to gain air superiority or air supremacy as a prelude to launching Operation Sea Lion, an amphibious assault by the Heer (German Army) onto the British mainland. Advised that the Kriegsmarine (German Navy) would not be ready until mid-August, Hitler wanted to mount the invasion on or around 25 August. Much later and weather conditions would not be favourable.
The Battle of Britain began on 10 July, when the first Luftwaffe bomber fleets began attacking convoys and Royal Navy forces in English ports and the Channel. The results were positive and the Germans succeeded in forcing the British to redirect shipping to ports in north-eastern Britain. The Luftwaffe then began the second phase, codenamed Unternehmen Adlerangriff ("Operation Eagle Attack"), of its air offensive, attacking RAF airfields and supporting structures on the British mainland. On 12 August, it flew the first missions of this phase. The next day, the Luftwaffe carried out its largest attack to date on the mainland. Christened Adlertag ("Eagle Day"), the attack was not a success and the RAF airfields damaged were in action again within hours.
Hitler was dissatisfied with the Luftwaffe's lack of progress and had already postponed Operation Sealion to late September after being advised the Kriegsmarine needed more preparation time. Prompted by an RAF raid on Berlin in late August, he ordered the Luftwaffe to concentrate its attacks upon London. It was thought the move would draw Fighter Command into a large, decisive battle. Initially, this strategy caught the British off-guard. The first daylight attack of this type occurred on 7 September and caused extensive damage to shipping and some 1,600 civilians were killed or injured. Over the next few days, bad weather prevented more large attacks. On 9 and 11 September, only smaller raids were carried out. The respite gave Air Chief Marshal Hugh Dowding, commander of RAF Fighter Command, the chance to prepare and reinforce his forces.
German strategy
On 14 September, Hitler and his command met to discuss the proposed invasion of England. Reichsmarschall Hermann Göring, the commander of the Luftwaffe, was not present at the meeting; he was inspecting Luftwaffe units in France. A subordinate, Generalfeldmarschall Erhard Milch, replaced him. Hitler praised the attacks which had caused heavy damage to the RAF and London, noting they may have been more effective had there been better weather. Nevertheless, he dismissed over-optimistic reports from the Oberkommando der Luftwaffe (OKL or High Command of the Air Force), particularly the Chief of the Luftwaffe general staff, General der Flieger Hans Jeschonnek, who asserted the RAF was on its last legs. The Luftwaffe did not have air superiority and due to this, Operation Sea Lion could not proceed for the time being. Großadmiral Erich Raeder, commander-in-chief of the Kriegsmarine, agreed and argued the invasion was a last resort regardless of gaining air superiority.
Hitler decided to defer his decision whether to commence Operation Sea Lion until 17 September. This gave four days for the Wehrmacht to prepare for the next phase of favourable tides commencing on 21 September. In the meantime, to maintain pressure on Britain, Hitler wanted to bomb military targets in London. However, Jeschonnek considered that this would not significantly affect morale among the civilian population. The primary residential areas of London were some distance from the military facilities that Hitler wanted the Luftwaffe to focus on. The bombing of population centres was considered, but Hitler refused to allow these to be attacked, for the time being at least.
Planning
To date, the Luftwaffe's attacks on London appeared to suggest to German intelligence that the RAF was close to collapse. The bomber crews did not encounter the robust defence that had characterised the battles in August. By striking against vital choke points in London that the RAF would be forced to defend, the Luftwaffe had the opportunity to destroy the remaining RAF fighters. This would achieve the desired air superiority, and eliminate a vital rail network, destroy shipping and supplies brought in from North America, and affect civilian morale by demonstrating London's vulnerability to air power.
A period of good weather was predicted for France, Belgium and southern Britain on 15 September. Accordingly, the Luftwaffe prepared for an attack on this date along the lines set by Hitler. Staff officers of Luftflotte 2 based in Brussels began planning for a two-pronged attack. The targets for the first prong was the network of railway tracks, which in some areas were 12 abreast and included "choke points" in the form of vulnerable rail-over-rail bridges, in the Battersea district. These tracks linked London to the West Midlands and other industrial cities in the north of Britain. The target for the second attack, which was to be larger than the first and scheduled for the afternoon, was the dock areas of the Thames Estuary including the warehouses of the East End of London, Surrey Commercial Docks, south of the river, and the various Royal Docks of the London Docklands, to the north.
The strike against London meant that most of the fighting on 15 September would take place between Luftflotte 2 under Albert Kesselring and Air Vice Marshal Keith Park's No. 11 Group.
British strategy
No. 11 Group covered the southeast corner of England, from a line extending northwest from near Southampton, on the south coast, to Aylesbury, in Buckinghamshire, and then approximately northwest across to Lowestoft. The group's sector stations were Northolt, to the west of London, North Weald and Debden to London's north with Debden closer to No. 11 Group's boundary with its northern neighbour, No. 12 Group, and Hornchurch to London's east. Biggin Hill and Kenley were to the south of London.
Dowding saw No. 12 Group in the role of the protector of the Midlands and a reserve for No. 11 Group. However, Air Vice Marshal Trafford Leigh-Mallory, commanding No. 12 Group, was frustrated with the way in which his squadrons were being used in the Battle of Britain. During the Luftwaffe attacks on southeast England, the squadrons of No. 12 Group were tasked with protecting No. 11 Group sector stations north of the Thames Estuary. However Leigh-Mallory believed his units should be in action south of the Thames. He was also critical of the way Park and Dowding were conducting the battle.
Squadron Leader Douglas Bader, commander of No. 242 Squadron, one of the units of No. 12 Group, was keen for a greater involvement in the fighting to the south. With the support of Leigh-Mallory, he advocated scrambling No. 12 Group squadrons as soon as German aircraft were detected forming up over France or Belgium, arguing that the RAF fighters were quick enough to reach 20,000 ft (6,100 m) over the Thames Estuary before the enemy reached the area. He planned to use three to five squadrons in a 'Big Wing' formation to engage the enemy. Should this succeed, No. 11 Group, following up attacks, might have found broken enemy formations whose crews had lost the determination to press on to their targets. Bader implied that this might reduce the losses of fighter pilots in No. 11 Group.
Leigh-Mallory wanted to ignore the protective and reserve role envisaged by Dowding for No. 12 Group in order to commit it to battle. While this method might have spared No. 11 Group, the Big Wing tactic had its problems. The amount of time it took to position such a large formation for interception meant that the Big Wing often failed to prevent bombing raids on No. 11 Group's airfields. Instead, they engaged the bombers as they withdrew. Leigh-Mallory accepted this and argued that it did not matter when an interception was made, as long as it accounted for a large number of enemy aircraft. The rationale was that the resulting losses, expected to be significant, would act as a deterrent. Damage sustained on an occasion when the Germans did get through would have to be offset against later occasions when they did not even care to try.
Park, with endorsement from Dowding, took the view it was unimportant to inflict large losses on the Germans in comparison to safeguarding his own forces. The airfields themselves supported the squadrons in the field, while the loss of the vital sector-stations could well have crippled the defence system. Thus the possibility of allowing the bulk of the German bombers to reach their target unscathed was unacceptable. Park believed the Germans would give up if they could not achieve their aim of air superiority. This meant avoiding the destruction and/or depletion of Fighter Command, as it was the primary factor in England's air defence. This would be achieved by sending small numbers of fighters to intercept, minimising losses in the air. By remaining to offer undiminished and constant air opposition, the RAF ensured the Luftwaffe could not win. As long as some sort of cost was imposed before the enemy dropped his bombs and impaired the defence system, the RAF could remain intact to meet the threat again the next day. This strategy would encourage an enemy to give up if he felt he was getting nowhere. Even while enemy losses remained moderate, it would be senseless to suffer those casualties for no return. Park and Dowding's strategy, under the circumstances, was the wiser choice.
Forces involved
Luftwaffe forces
The Luftwaffe had suffered heavy attrition since the beginning of the Battle of Britain. Just over a month earlier, on 17 August, it had possessed 2,226 operational aircraft. By 7 September, it had 1,895 aircraft, a drop of 15 percent. Still, most of the losses were being made good by production. During the battle, the Luftwaffe had undergone a major reorganisation. Luftflotte 5 in Norway had sent most of its Messerschmitt Bf 110 and medium bomber units (Kampfgeschwader or Bomber Wings) to Luftflotte 2 and 3. Luftflotte 3 then passed most of its Messerschmitt Bf 109 units to Luftflotte 2, which was based in the Netherlands, Belgium and France.
According to the Luftwaffe order of battle dated 7 September, the nearest date covered by the list, the three Air Fleets contained 1,895 aircraft. Luftflotte 2 had 1,311 machines, including 533 Bf 109s, 107 Bf 110s, 51 reconnaissance and 484 medium bomber aircraft. A further 120 Junkers Ju 87 dive bombers were on the order of battle, but were not used.
Luftwaffe readiness was less than ideal. In August, 211 pilots had been killed, including 105 fighter pilots and 91 bomber pilots. Missing pilots amounted to 132 fighter and 94 bomber pilots alone, with a further respective loss of 47 and 28 wounded. The effect on operation-ready crews was significant. Messerschmitt Bf 110 units had 60% of crews against authorised strength. For bomber units, it was 65%, while Bf 109 units had 81% of crews ready, a 5% increase from the 76% level in the first week of September. However, by 14 September, Bf 109 units possessed only 67% of crews against authorised aircraft. In Bf 110 units, it fell to just 46%, and in bomber units it dropped to 59%. One week later, the figures were 64, 52 and 52% respectively.