Dunkirk - The Miracle of a Lifetime

The Mistake.

The French borders were very well-defended, one of the examples of the defenses were the Maginot Line, oh yeah, that's the only defensive line the French built before the war.
The British asked permission from Belgium so their troops can make defenses, but Belgium declined, but when the Germans took siege of Belgium 3 hours later, the Belgians allowed British troops to enter, and hundreds of thousands of troops rushed through the streets to support the Belgian Army.
But, a small weakspot is shown after Holland is occupied.
The crowded forest, The Ardennes.
The French thought it would be impenetrable, so they made very light defenses.
But the German Army Group B, consisting of thousands of tanks, pushed through the lines quickly and outflanked the British soldiers. While the south of France, becomes exposed to attacks.
The Germans swiftly invade, while the French defenses fail to hold the Germans off, so Prime Minister Winston Churchill proposes a plan, which saved over 300,000 troops...

Operation Dynamo.