From Walter Lord’s book, The Miracle of Dunkirk, the moment that Winston Churchill realized that the Allied forces situation was worse than imagined:

At 7:30 a.m. on May 15, Churchill was asleep in his quarters at Admiralty House, London. The bedside phone rang; it was French Premier Paul Reynaud. “We have been defeated,” Reynaud blurted in English.

A nonplussed silence, as Churchill tried to collect himself.

“We are beaten”; Reynaud went on, “we have lost the battle.”

“Surely it can’t have happened so soon?” Churchill finally managed to say.

“The front is broken near Sedan; they are pouring through in great numbers with tanks and armored cars.”

Churchill did his best to soothe the man—reminded him of the dark days in 1918 when all turned out well in the end—but Reynaud remained distraught. He ended as he had begun: “We are defeated; we have lost the battle.”

Later that day, Churchill’s letter to Roosevelt notes the following:

… We expect to be attacked here ourselves, both from the air and by parachute and air borne troops in the near future, and are getting ready for them. If necessary, we shall continue the war alone and we are not afraid of that. But I trust you realize, Mr. President, that the voice and force of the United States may count for nothing if they are withheld too long. You may have a completely subjugated, Nazified Europe established with astonishing swiftness, and the weight may be more than we can bear.

Bombardier Watkin went from thinking the Allied forces were on the verge of a victory the previous day, to making the following entry in his diary:

What a day! We are due to retreat at 10:30 p.m., and as we do, we get heavy shellfire, and we thank God we are all safe. … Except for the shock I am o.k.

May 15th was Winston Churchill’s 6th day as Prime Minister.

Walter Lord summaries why all were so shocked:

It seemed incredible. Since 1918 the French Army had been generally regarded as the finest in the world. With the rearmament of Germany under Adolf Hitler, there was obviously a new military power in Europe, but still, her leaders were untested and her weapons smacked of gimmickry. When the Third Reich swallowed one Central European country after another, this was attributed to bluff and bluster. When war finally did break out in 1939 and Poland fell in three weeks, this was written off as something that could happen to Poles—but not to the West. When Denmark and Norway went in April 1940, this seemed just an underhanded trick; it could be rectified later.

Then after eight months of quiet—“the phony war”—on the 10th of May, Hitler suddenly struck at Holland, Belgium, and Luxembourg. Convinced that the attack was a replay of 1914, the Supreme Allied Commander, General Maurice Gamelin, rushed his northern armies—including the British Expeditionary Force—to the rescue.

But Gamelin had miscalculated. It was not 1914 all over again. Instead of a great sweep through Flanders, the main German thrust was farther south, through the “impenetrable” Ardennes Forest. This was said to be poor tank country, and the French hadn’t even bothered to extend the supposedly impenetrable Maginot Line to cover it.

Another miscalculation. While General Colonel Fedor von Bock’s Army Group B tied up the Allies in Belgium, General Colonel Gerd von Rundstedt’s Army Group A came crashing through the Ardennes. Spearheaded by 1,806 tanks and supported by 325 Stuka dive bombers, Rundstedt’s columns stormed across the River Meuse and now were knifing through the French countryside.