Even before the rescue of the British Expeditionary Force from Dunkirk (Operation Dynamo) began, Winston Churchill foresaw the worse-case scenario that could follow from the encirclement of the doomed Anglo-French armies. France herself might well go down in defeat, so leaving a half-armed Britain alone to confront triumphant German armed forces. That would make it all too likely that Hitler would decide to put an end to the war by invading England.
So on 26 May 1940 Churchill asked the Chiefs of Staff a direct question: "Can the Royal Navy and the Royal Air Force hold out reasonable hopes of preventing serious invasion?"
The Chiefs of Staff replied that as long as the Royal Air Force remained "in being", then the Royal Navy and the air force between them "should be able to prevent Germany carrying out a serious sea-borne invasion of this country". But if Germany obtained air superiority, then the Navy could hold off an invasion "for a time", but not "for an indefinite period". Once a large-scale invasion had been launched, Britain's land defences would not be strong enough to prevent the German army establishing a firm bridgehead, nor from subsequently heading inland. Therefore, concluded the Chiefs of Staff, "the crux of the matter is air superiority".
Hitler's generals and admirals came to exactly the same conclusion when, at the end of July, the Führer issued his directive for "Operation Sealion", a cross-Channel invasion of south-east England. They reckoned that, without German mastery of the skies, the Royal Navy could play havoc in mid-Channel with Sealion's unwieldy mass of river barges stuffed with troops and equipment. Such a venture must inevitably end in catastrophe.
So when Reichsmarschall Hermann Göring launched the Luftwaffe into its daylight offensive against the Royal Air Force and its bases in south-east England on 13 August (Adler Tag, or "Eagle Day"), the leaderships on both sides recognised that the fate of "Sealion" – the fate of Britain, indeed – turned on the outcome of the battle. And the British nation recognised it, too. In the words of a newspaper vendor to a customer: "Well, we're in the final, and it's on the home ground."
As I can remember, as a schoolboy in south London, there was no dismay among my family and their friends at the sight of contorted vapour trails high over us as Fighter Command and the Luftwaffe fought it out in the blue summer sky – only a sense of excitement. Looking back now as a historian, it is clear to me that in 1940 the British nation was blessed by an inner certainty that, just as the Navy had seen off Philip II of Spain in 1588 and Napoleon in 1805, so now the Royal Air Force and the Navy together would see off that funny little man with the toothbrush moustache and his fat chum in the gawdy uniform covered in medals. In that certainty, there was truly an element of the heroic.
Indeed, the whole nation followed the course of the battle, on the BBC Home Service radio and in the newspapers, as if it were a kind of super cup-tie, complete with each day's score.
Yet, in reality, there was no certainty at all about the outcome. When both sides' relative strengths in numbers, tactics and equipment are balanced out, they were very much of a match.
Each began the battle with some 900 operationally available single-seat fighters. The top speeds of the German Messerschmitt Bf 109 and the British Supermarine Spitfire were virtually the same, at 354mph and 355mph, although the Hawker Hurricane was slower, at 329mph. The British fighters were armed with eight Browning machine guns mounted within the wings; the Bf 109 with two 20mm cannon and four machine guns. While the Spitfire was more agile than the Bf 109 in a dogfight, the German fighter could climb faster and higher. The slower but robustly built Hawker Hurricane made a very stable gun platform: its role in the battle was to shoot down German bombers while the Spitfire dealt with the Bf 109s.
Yet it was the young pilots of Fighter Command who passed into British myth as "The Few" who outfought vast German airfleets. Today, 70 years on, we can acknowledge that the young men in the Messerschmitts were just as gallant, high-spirited and skilful. But whereas the German pilots were fighting for a hideous tyrant in the delusion that they were patriotically defending the Fatherland, the pilots of Fighter Command were modern-day Spartans, holding the pass for the free world against the barbarian. They included volunteers from the British Dominions overseas, from countries under Nazi occupation such as Czechoslovakia and Poland, and even a handful from neutral America.
If pilots and aircraft were broadly equivalent in numbers and combat effectiveness, what accounted for the ultimate British victory?
Certainly Göring committed a major blunder when he switched the Luftwaffe's attacks from Fighter Command's sector stations to raids on London, and so accorded Air Marshall Sir Hugh Dowding, chief of Fighter Command, a respite. But the essential key to the British victory lay in Fighter Command's sophisticated intelligence-gathering system and centralised control of the battle, compared with the Luftwaffe's Biggles-like operations off forward airfields in northern France and Flanders.
On 14 July 1936 Dowding had assumed command of a new air defence organisation for the United Kingdom – Fighter Command. It was the year after R A (later Sir Robert) Watson Watt had bounced radio waves from the BBC transmitter at Daventry off a Heyford bomber, so demonstrating that aircraft could be detected at long distances by radio-location ("radar"). By the summer of 1940 the lattice radar masts of the 21 "Chain Home" system had been erected along the east and south coasts of England, its range extending to 120 miles, far enough out to cover German forward airfields.
Yet "Chain Home" formed only one essential ingredient of Dowding's concept of future air battles not as free-wheeling Western-Front-style dogfights, but, like land or sea battles, fought under the central direction of the commander-in-chief.
The key to Dowding's new organisation lay in Fighter Command's headquarters at Stanmore in west London. Here intelligence from radar stations and the 30,000-strong Observer Corps about the location of incoming German raiders was processed and plotted. Counters moved by members of the Women's Auxiliary Air Force (Waafs) across a huge map table gave Dowding and his staff an unfolding picture of German penetrations and the defensive deployment of British fighters.
Up-to-date intelligence would then be fed down to Fighter Command's group and sector commanders who, under Dowding's strategic direction, were conducting the tactical battles.
Time and again, the Luftwaffe bafflingly found the Spitfires and Hurricanes waiting for them at the right place and at the right time. Moreover, radar and sophisticated organisation also enabled the RAF to judge with fair accuracy the number of aircraft needed to fly in order to deal with a particular raid. In this way, Fighter Command could husband its resources. Here was another key to victory in what was essentially a battle of sheer attrition – the loss of aircraft and of that scarcer resource, pilots.
Thanks also to the steady output of aircraft from the factories, and of pilots from the training units, Fighter Command actually had more aircraft and pilots available when the battle ended in mid-September than on "Eagle Day" in July.
The Luftwaffe had lost over 1,700 aircraft, plus 640 damaged. On the famously climatic day of the battle, 15 September (now Battle of Britain Day), 60 of its aircraft were shot down. In the face of such cumulative losses, the Luftwaffe could not continue its daylight offensive, and turned to night-bombing of Britain's cities instead – "the Blitz".
The Battle of Britain was the first decisive victory in history won by airpower alone. By ensuring Britain's survival, it made possible the huge deployment from 1941 onwards of American military power in the British Isles. And that deployment in turn made possible the Anglo-American invasion of Hitler's Europe in June 1944.
Fighter Command's triumph had other consequences. Because Hitler had been robbed of his hoped-for final victory in the West, he turned to the East, and in June 1941 invaded the Soviet Union. But instead of the expected short and successful campaign, he had begun a conflict in which his forces would be gutted in four years of mass battles with the Red Army: ending on the streets of his own capital, Berlin.
Given Sir Hugh Dowding's achievement, how was it that he was removed from Fighter Command just two months after his victory? What personages in high places had he upset with his well-known forthrightness? We shall never know.
Correlli Barnett is Fellow of Churchill College Cambridge and author of The Audit of War (Faber)