In his latest war book, The Secret War, Max Hastings compares the intelligence gathering efforts of both sides in the second world war and concludes that, whatever the Allies’ blunders, their rivals in Hitler’s Abwehr were “incomparably worse”.

Even as German-led Europe staggers through its protracted eurozone crisis, German ineptitude remains a concept we struggle to acknowledge. Yet it is central to their conduct of the Battle of Britain, whose fateful climax we (but not they) are about to celebrate.

The British made mistakes, too. Unlike the runup to 1914, when lurid invasion scares were the stuff of military planning, bestsellers such as The Riddle of the Sands and even West End plays, the prospect had been widely ignored throughout the 1930s until the miraculous evacuation of the army from Dunkirk in May 1940.

Then it suddenly stared everyone in the face. As hasty improvisations were made, including the new Home Guard of later Dad’s Army fame, the rumour mill spun wildly. German parachutists (they might be dressed as nuns) were reported, and thousands of dead bodies allegedly spotted in the Channel. It would all have put Twitter to shame. The likelihood of a real invasion between Dorset and Kent depended on control of the airspace and the “fighter boys” – teenagers, some of them – whom Winston Churchill famously dubbed the Few on 20 August as the battle raged.

Germans are half right, then as now, when they dismiss the battle as a sideshow. Compared with the assault by 180 divisions on Soviet Russia that Hitler was already planning for the following spring, the aerial battle fought mostly over southern England 75 summers ago was just that.

Winston Churchill in 1940. Photograph: Keystone-France/via Getty Images

But Churchill’s assessment was more strategically sound. As Britain’s only recently installed prime minister, he used his epic “finest hour” speech to the Commons on 18 June (the day after France sought an armistice) to predict that “Hitler knows he will have to break us in this island or lose the war.”

He didn’t, and he did.

Was that a grandiose and self-serving judgment by a leader who knew he had to raise shaky morale (he included a few jokes) at all costs? There must have been an element of that. But Hitler’s insecurity (his generals called it his “island madness”) would make him keep large garrisons and weapons on the barricaded coasts of occupied northern Europe, so that even the irrelevant Channel islands were turned into a fortress.

It all helped weaken the nearly successful German attack on Russia in June 1941 and kept Berlin exactly where it never wanted to be: in a two-front war. It hardly seems likely that the United States, like Russia a victim of surprise attack (by Japan) six months later, could have prioritised the European war and embraced the Anglo-American D-day invasion in June 1944 if Britain had given up the fight as Hitler decided it logically must in 1940.

Critics of the western powers often point out Soviet Russia took the brunt of Hitler’s ambitious plans (and had 20 million war dead to prove it), although Stalin had been far more naively complicit in the outbreak of war in 1939. But, after its initial humiliations, the military might of the Red Army could well have prevailed unaided in the ruins of Berlin, as it did in 1945. But that would have ushered in a very different future for western Europe. Not a happy thought.

Back to the Battle of Britain sideshow. With the sudden collapse of France after a mere five weeks of real war – so different from 1914 – the Führer assumed that Britain would recognise that its military position since Dunkirk had become hopeless. It would sensibly negotiate an armistice, as the foreign secretary, Lord Halifax, wanted to do, but the Churchillians and the Labour party did not.

What Hitler did was therefore designed to secure that end: a deal, not a full-scale invasion. Fighter Command would be destroyed – flushed with his European successes, ex-fighter ace turned military chief Hermann Göring said it could be done in four days – and a fleet of barges assembled at French channel ports. Hopefully mere bluff would do the trick, as it had against Austria in 1938.

So it was only in May that Hitler had discussed the invasion options with his not very keen admirals, only in mid-July that he issued directive number 16: “I have decided to begin to prepare for and, if necessary, to carry out an invasion of England.”

If that sounds tentative, it was. Operation Lion (Hitler himself changed it to “Sealion”) was intended to provoke British panic, disintegration and capitulation. A few days later, he offered peace terms to Herr Churchill in a Reichstag speech, which said he had no wish to “destroy a great empire”. On the night of 1 August, leaflets entitled “A Last Appeal to Reason” were airdropped all over Britain. Hitler felt ambiguous towards “Anglo-Saxon” Britain, admiration mixed with a determination to humiliate.

Two Dornier 17 bombers over West Ham, London, on 7 September 1940. Photograph: IWM /Getty Images

But the Britain of 1940 (the “keep calm and carry on” era) was largely unmoved. Trenches were dug, children and artworks moved to safety, Home Guard units formed and woefully underequipped. Sites for mass graves were agreed, beaches mined and gas masks worn. Fear of what bombers could do had been amply borne out in Spain and, more recently, in France and Poland.

But a network of resistance units was also formed to commit sabotage behind the invaders’ lines. Peter (brother of Ian) Fleming, who was in charge of such units and later wrote a good book, Operation Sea Lion, about it, concluded that the mood was determined but quite innocent: after all, an invasion like this had not happened here since 1066.

Despite both sides being aware of Spanish (1588) and French (1803 etc) plans to conquer England, the Germans, who were not notably seafaring (Hitler feared the sea), miscalculated, too. A limited invasion was regarded as not fundamentally different from a major river crossing.

That was one critical error not shared by glum German naval planners. The sea is not like a river and the Channel in August can experience anything from bright sunshine to fog and storms.

Even without adequate air support, the Royal Navy would wreak terrible havoc on the troop barges as they ploughed west (in theory through a minefield corridor) on a very wide front. The German navy, heavily damaged in the recent Norway campaign (it lost two cruisers and 10 destroyers), expected their British counterparts to throw everything they had at an invasion. And, in 1940, that was a lot.

Meanwhile, it was up to the fighter boys in their Spitfires and underpraised Hurricanes to hold the line of vapour trails over Kent. In seeing off the enemy, the heroes of the hour, boozy and glamorous (Poles, Canadians, Czechs, Kiwis included) also broke traditional class barriers that previously characterised Britain’s military elite. The shortage of replacement planes proved less of a problem than a shortage of trained and experienced pilots. It was worse for the Germans, who were fighting over enemy territory.

Flying Officer Nevill Duke of No92 East India Squadron, photographed by Cecil Beaton. Photograph: IWM

As everyone knows, more or less, the Luftwaffe came close to overwhelming the RAF several times in that hot summer, not least when it attacked the visible and vital radar masts at key airfields on 12 August. Puffed up by overly optimistic reports about damage done and puzzled by British resilience (the radar system was much better than they realised), Göring failed to stick to winning ways. Unable to resolve the rival claims of his bomber and fighter chiefs for priority, his frequent changes let the RAF off the hook more than once.

The greatest strategic error arose from a chain of accidents. German fighters had been escorting the bombers, often at great cost in terms of lives and aircraft, but Hitler had told them not to bomb London except on his express orders. Then, in late August, the British capital was accidentally bombed in mistake for military targets. The RAF duly bombed Berlin in retaliation on 25 August – something Germans had been assured would never happen.

Hitler was furious. In a speech on 4 September, he mocked the British for asking “Why doesn’t he come?” “Calm yourselves, calm yourselves, he is coming,” he said, to tumultuous applause.

But he wasn’t coming. As usual, he was misreading his enemy, too. An offer to repatriate 40,000 PoWs might have worked better.

Berlin switched its full attention to daylight raids on British cities on 7 September, with 400 bombers and 600 fighters attacking key targets such as the London docks. The Blitz had begun. But the attacks on airfields ended and Fighter Command was spared to fight on, heavy losses rapidly forcing the Luftwaffe later to confine itself to night bombing raids.

With both sides exaggerating their kill rate (easily done in the fog of war), the air battle’s climax came on 15 September – now Battle of Britain day – when the Germans lost what is now said to be 60 planes to the RAF’s 26. Two days later, Hitler postponed Operation Sealion until the following spring – ie, indefinitely. He wasn’t coming after all.

The Battle of Britain was the last decisive occasion on which British military forces, acting more or less alone, had a decisive impact on the course of world history: it mattered that we fought on. But very soon, two emerging superpowers would come to dominate as near-bankruptcy forced Britain into imperial retreat and towards domestic priorities, like the welfare state, more valued by its war-weary people.

No one now active in public life has any direct memory of those events, though some are still alive. Churchill said at the time: “Hitler has not yet been withstood by a nation with a willpower equal to his own.” It is sometimes unclear, 75 years on, whether that nation retains the willpower to do more than stagger on to next week. That is surely the wrong conclusion to draw. The class of 1940 was no better or worse than us, but they were lucky. Nicer weather, too.