Here is a question that sounds like a riddle. What is the problem with a wall?
The answer is that a wall has to stop somewhere.
In the nineteen thirties, France built a wall along its border with Germany. It was not a wall in the way you are picturing. It was the most extraordinary piece of engineering anybody had ever attempted, and the story of what happened to it is the story of how France fell in six weeks.
It was called the Maginot Line, after the government minister who pushed it through, and it cost France an amount of money that is genuinely hard to convert into anything meaningful today. Think of it as several years of the entire defence budget, poured into concrete.
Above ground you would have seen very little. A low dome here. A slit in a hillside there. Almost all of it was underground.
Underneath were fortresses that went down several storeys. They had their own electric power stations and their own air supply, filtered in case of gas. They had underground railways to move ammunition from one gun position to another, some of them miles long, with proper little trains. They had kitchens, hospitals, dormitories, and in at least one of them, a cinema.
The guns could be raised out of the ground, fired, and lowered again. The armour was designed to survive a direct hit from the heaviest artillery in existence. Soldiers who served in it said the strangest thing was the quiet, and the fact that you had no idea what the weather was doing.
As a piece of engineering it was magnificent. As a piece of defence, it did exactly what it was built to do. The German army never seriously attacked it.
The Maginot Line ran along the German border. Then it reached the corner where France, Germany, Luxembourg and Belgium meet, and it more or less stopped.
There were reasons. Building it onward along the Belgian frontier would have cost as much again, and money had run out. The ground in the north is wet and flat and hard to tunnel. And there was a diplomatic problem that mattered more than either: Belgium was an ally. Walling yourself off from your ally sends a fairly clear message about how long you expect to be defending them.
So the plan was that if Germany came, they would come through Belgium, as they had done in nineteen fourteen, and the best French and British armies would rush north to meet them. Everybody understood this. It had been rehearsed.
Between the end of the wall and the flat ground of Belgium lay one more stretch of border, about a hundred kilometres of it, covered in steep hills and thick forest and threaded by narrow roads with sharp bends. This was the Ardennes.
It had been studied. The conclusion of the study was that an army could get through it, but only slowly, and that the French would have plenty of time to react. A tank column in there would be strung out along single roads with no room to turn, which is about the most helpless thing a tank column can be.
That conclusion was correct. It was also the whole plan.
The idea of pushing an armoured force through the Ardennes did not come from the top. It came from a staff officer called Erich von Manstein, and when he first proposed it, his superiors did not like it at all.
This is worth sitting with for a second. The plan that broke France in six weeks was rejected repeatedly by the German high command, who thought it was reckless, and who eventually got so tired of hearing about it that they moved Manstein to a different job to stop him raising it.
It only survived because of an accident. In January of nineteen forty, a German aeroplane carrying an officer with a copy of the original invasion plan got lost in fog and crash-landed in Belgium. The officer tried to burn the documents in a hedge and did not entirely succeed.
The Allies recovered enough of them to see the outline of the expected attack. The Germans knew the Allies had them. Which meant the old plan, the one the high command preferred, was now useless.
So they went looking for another one, and there was Manstein, still saying the same thing about the forest.
On the tenth of May, nineteen forty, German forces attacked the Netherlands and Belgium. It looked exactly like the expected move. The best Allied armies drove north to meet it, and they drove fast, and they drove with confidence, because this was the situation they had prepared for.
That attack was real, but it was not the main one. It was there to be chased.
The main force went into the Ardennes. Seven armoured divisions, plus everything that supports them, plus the fuel to keep them moving. Something over forty thousand vehicles, on a handful of forest roads.
What resulted was one of the most extraordinary traffic jams in history. The column backed up, by some accounts, two hundred and fifty kilometres, which means the tail of it was still inside Germany while the head was coming out of the trees in France. Officers stood at junctions directing lorries like policemen. The whole thing crawled.
It was also, for several days, the most inviting target that has ever existed. A few squadrons of bombers over those roads would have turned the invasion of France into a very different story.
Nobody came. Aerial reconnaissance did spot movement in the Ardennes and reported it. The reports went up the chain and were not believed, because everybody knew that a major armoured force could not operate in the Ardennes. The reports did not match the picture, so the picture won.
The German columns came out of the forest at the river Meuse, near a town called Sedan, on the thirteenth of May. They crossed it that day.
Then they turned, not south towards Paris as everyone expected, but west. Straight for the sea.
The plan had a name that describes it perfectly. The Germans called it the sickle cut. The Allied armies had rushed north into Belgium, and now an armoured line was being drawn behind them, across their supply routes, between them and the rest of France.
They moved so fast that their own high command panicked and twice ordered them to slow down and wait for the infantry to catch up. One divisional commander, a man called Rommel, moved so far ahead of everyone that his own headquarters lost track of where he was, and the newspapers started calling his unit the Ghost Division.
On the twentieth of May, ten days after the attack began, German tanks reached the English Channel at the mouth of the Somme.
Everything north of that line was cut off. That included almost the entire British army, and the best of the French one, and they were now standing in Belgium with the sea behind them.
Which is where the next episode begins.
People often say the French were foolish to build the Maginot Line, or that they were cowards hiding behind concrete. Neither is true, and the second is unkind. The line worked. It was never broken. The problem was not the wall, it was the assumption printed underneath it: that the ground at the end of the wall was impossible.
That assumption was not lazy either. It was the result of proper study by serious people. It was simply out of date. Tanks and radios in nineteen forty could do things that tanks and radios in nineteen thirty could not, and the study had not been redone.
The most important failure in this whole episode is not a military one. Pilots saw the columns. They reported them. Officers read the reports, compared them against what everyone knew to be true, and decided the reports must be wrong.
Here is something to think about tonight. When a piece of information does not fit what you are certain of, you have two choices: change your mind, or decide the information is faulty. Almost everybody, almost always, picks the second. What would it take to make you pick the first?