Shocking gamble Nazi forces took in WW2 to attack France, Belgium and Luxenbourg

For months, British soldiers dug trenches, drank tea, trained and peered warily to the east, but nothing of note ever happened. The newsmen called it the "Phoney War", the "SitzKreig" and even "the Bore War". Then suddenly, on May 10, 1940, boredom was replaced with horror and shock, leaving the entire world astonished and heralding the resignation of Neville Chamberlain and the arrival of Winston Churchill as PM.

Germany's invasion of Holland, Belgium, Luxembourg and France was one of the most dramatic and successful military operations of all time - yet it was also a spectacular gamble. Compared to the French and British, Hitler's generals commanded fewer divisions (135 against 155), fewer tanks, fewer warplanes and about half the number of artillery guns.

France had the world's biggest army and had been preparing to defend its borders for many years. It had been joined by a smaller but highly professional British Expeditionary Force (BEF) supported by the world's most powerful navy.

So how did everything unravel in just six panic-stricken weeks? Primarily it was because the French had prepared brilliantly... to fight a different kind of war, a war of concrete, wire and artillery - the war of 1914-1918. They had spent millions of francs building a formidable series of defences called the Maginot Line along the Franco-German border. Newspapers were full of photographs highlighting its magnificent engineering and it was commonly believed to be impregnable.

And along with their static gun emplacements came a fixed, top-down military doctrine ideal for the attritional grind of the previous war.

Unfortunately this was not the war that the German military wanted to fight. Instead, Hitler's commanders had pioneered a new offensive style, something that today we'd call "combined arms warfare".

Infantry, armour and aircraft - linked by instant radio communication - would work together, striking and moving swiftly, isolating enemy strongpoints and hitting deep behind the lines to cut communication and supply chains, keeping their enemies off-balance.

The attack began in Belgium and Holland. This drew the British North and East to previously established defensive lines along the River Dyle. But it was a trap, and it snapped shut the moment the main German offensive suddenly roared into life with the sound of a thousand Panzer tank engines many miles to the south in the hills and forests of the Ardennes. Here, straddling the borders of France, Luxembourg and southern Belgium, there were no impressive Maginot Line installations - just a few second-rank units of the French army.

This was largely because the Commander in Chief of the French armed forces, General Maurice Gamelin, had called the region "Europe's best tank obstacle". He believed only a fool would try and move an armoured force along its twisty woodland tracks. But move through it the Germans did - against light opposition and at impressive speed.

It soon became clear that if the Germans could get through the Ardennes they could ignore the giant forts of the Maginot Line altogether and simply drive to the English Channel or to Paris - or to both.

Although frequently outnumbered, the elite Panzer and mechanised units were energetic and confident. And they were supported by ground-attack aircraft, notably the Stuka dive bomber, that commanders on the ground could summon quickly on their radios.

The French - because they had expected a static, defensive war - were reliant on dispatch riders and runners. As the battle raged in the hills, German bombers hit French and RAF air bases deep inside France - and there was no radar coverage to give the Allies any advance warning of an incoming attack. By May 13, there was widespread chaos in the French military, and the German army was poised to break out from the Ardennes to the flatter lands that stretched west and south - ideal tank country.

Many hurtful myths about supposed French cowardice emerged from these desperate days - jokes about reverse gear tanks and white flag factories. In fact the sons of the men who had stopped the German army in 1914 resisted bravely when they could but, in the face of this new kind of warfare, a dark fatalism - even defeatism - soon spread.

Outmanoeuvred and outfought by suddenly-appearing tanks and diving Stukas heralded by terrifying sirens attached to their airframes, units became disconnected, disoriented and demoralised. Soon, columns of desperate refugees added to the chaos and congestion on the roads.

On May 14 the key French army commander General Alphonse Georges announced to his staff that the Ardennes was lost, and then collapsed weeping into an armchair. Some French politicians - to the outrage of new British Prime Minister Winston Churchill - already looked and sounded like defeated men.

As the German tank columns raced across northern France, the French airforce threw itself bravely, even foolhardily, into the battle, bombing every bridgehead over the River Meuse. But they were hammered by intense anti-aircraft fire and prowling Messerschmitt fighters. To help its ally, the RAF sent Britain's slow, lumbering and lightly armed bombers into the same "Battle of the Bridges", but they met exactly the same fate.

In just 24 hours, 40 Blenheims and Fairey Battles were lost from a force of71 attacking planes - one of the darkest days in the history of the RAF.

Shock at the speed of the German advance was supercharged when images of Rotterdam on fire spread around the world. Holland had been neutral during the Great War and hoped to be so again but Hitler had other ideas and the fighting lasted barely a week. The under-prepared Dutch army all but collapsed and Queen Wilhelmina fled to London by Royal Navy destroyer.

The German military - and especially its air force - suddenly seemed invincible and overwhelming. Years of well-made Nazi propaganda had seeded the idea that Hitler had readied a new kind of super soldier and an air force capable of levelling a city. The fear this now generated was one of his most potent weapons. French units began to surrender on a large scale.

Years later, I interviewed Peter Vaux, a young officer who in 1940 commanded a Matilda light tank at Arras. He vividly recalled: "We drove up a small slope through some scrub as fast as poor little Matildas could go, and then suddenly in front of us was a whole stream of German lorries and trucks and half-tracks, and they were as astounded, I bet, as we were."

Vaux drove right through the column, firing on everything in sight, and saw the enemy flee in panic. Knowing that they were running ahead of their own supplies - many of which were travelling slowly by old-fashioned horse and cart - the German commanders had always worried about a counter-attack like this.

But Rommel himself immediately fought back, turning his infamous 88mm anti-aircraft guns on the British.

Before he was captured, Vaux came across a field of smoking British tanks.

"They all had names. Devil, Dauntless, Dragon, and I knew who was in which tanks - they were my friends, you see, and they were lying dead all around and I felt absolutely shattered," he recalled.

Many historians believe the sacrifice made by Vaux's comrades at Arras quite possibly saved the British army in France - the surprise of it may have been one reason for Hitler's "halt order" that held the German forces back for three critical days and allowed the British Army and its French allies to create a defensible position around Dunkirk. But there was good reason for the Germans to be cautious. Defeating the British was to be welcomed - but Paris was the real prize.

The failure to take the French capital in 1914 haunted the Germans and, with their men exhausted and many tanks lost, they needed to ensure they had the resources to drive south. Anyway, the British could be left trapped on a beach - subject to devastating air attack from Hermann Goering's Luftwaffe.

The battle of Arras was a defeat that may have helped save an army but the other counter-attacks that were meant to accompany it were easily repulsed.

The Anglo-French alliance was cracking now, too, and on May 25 Prime Minister Paul Reynaud cabled Churchill complaining the British were not doing enough in the north. In return, Churchill complained about French failures and confided with some understatement to Neville Chamberlain that "if one side fights and the other does not then war is apt to become somewhat unequal". With Italy poised to enter the war on Hitler's side, Foreign Office civil servant Alexander Cadogan wrote in his diary: "Everything is complete confusion, no communications and no one knows what is going on except that it's black as black."

To emphasise the global scale of the shock, secret negotiations began in Canada between Mackenzie King's government and the US to determine what should happen if - or rather when - France and the UK fell under Nazi control. King confided to his diary that the world faced "not only the extinction of civilization in western Europe but its permanent destruction throughout the world".

France had bravely resisted the invader for four years in the previous war. This time it was all over in weeks and, on June 14, the streets of Paris echoed to the sound of a triumphant German army. Politicians in Washington and Ottawa were not the only ones imagining London might one day very soon witness the same thing.

• Phil Craig is author of 1945: The Reckoning, which has just been published by Hodder priced £25