Here is an excerpt from an article by Erik Sass for the Mental Floss Studios website. To read the complete article and check out others, please click here.

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With the release of Christopher Nolan’s critically acclaimed Dunkirk, the world’s attention is once again focused on the historic events recounted in the film, when a makeshift fleet of British fishing boats, pleasure yachts, and cargo ships helped save 185,000 British soldiers and 130,000 French soldiers from death or capture by German invaders during the Fall of France in May and June 1940. Here are five surprising facts about those heroic days.

1. The German attack was supposed to be impossible.

The main reason France collapsed so quickly in 1940 was the element of surprise enjoyed by its German attackers, thanks to General Erich von Manstein, who proposed an invasion route that was widely believed to be impossible. In Manstein’s plan, the main German column of tanks and motorized infantry would force their way through the forests of Ardennes in southeast Belgium and Luxembourg—a thick, hilly woodland which was supposed to be difficult terrain for tanks, requiring at least five days to cross, according to conventional wisdom based on the experience of the First World War. The French and British assumed that little had changed since the previous conflict, but thanks to field studies and updated maps, Manstein and his colleague General Heinz Guderian realized that a new network of narrow, paved roads would allow just enough room for tanks and trucks to squeeze through. As a result the Germans passed through Ardennes into northern France in just two-and-a-half days, threatening to cut off hundreds of thousands of Allied troops, with only one escape route: the sea.

2. One French word was burned into Winston Churchill’s memory: “Aucune.”

The German invasion of France began on May 10, 1940, the same day Winston Churchill became Prime Minister. By May 14, when he paid his first official visit to Britain’s ally, Holland had capitulated and Paris was preparing for evacuation. But an even worse surprise was in store. In one of the most famous passages of military history, Churchill recounted the moment he learned that the French didn’t have any troops in reserve:

“I then asked ‘Where is the strategic reserve?’ and, breaking into French … ‘Ou est la mass de manoeuvre?’ General Gamelin turned to me and, with a shake of the head and a shrug, replied. ‘Aucune.’ [There is none] … I was dumbfounded. What were we to think of the Great French Army and its highest chief? It had never occurred to me than any commanders … would have left themselves unprovided with a mass of manoeuvre … This was one of the greatest surprises I have had in my life.”

Here is a direct link to the complete article.