I haven't seen the movie but know enough of the story to understand that Allied troops had fallen back to Dunkirk for evacuation but were trapped there until transport came.
I also understand that the German army called off the invasion/annihilation of these troops for some reason.
Why did Germany not pounce on this easy opportunity to wipe out large numbers of Allied troops?
Although we know a great deal about the events surrounding Hitler's "Halt Order" at Dunkirk, the truth is that the reasons behind it are not completely understood by historians, even now.
It is a mistake, however, to think that the German army just stood around, watching the British Expeditionary Force being evacuated. They were fighting to reach the beaches the entire time the Allies were fighting to get off them. German artillery and aircraft shelled, bombed and strafed the troops on the beaches there without mercy.
Adolf Hitler's "Halt Order" actually just confirmed an order given by General Gerd von Rundstedt, commander of Army Group A (the main German force fighting in western France). In turn, von Rundstedt had issued his order at the request of his tank unit commander, who had lost almost 50% of his armoured forces and wanted to regroup. However, Hitler's "Halt Order" was more specific than von Rundstedt's. It specified that the line of Lens–Bethune–Saint-Omer–Gravelines "will not be passed".
This meant that some of the more advanced German units actually withdrew from positions that they had already taken. In particular, General Wilhelm von Thoma, Chief of the tank section of the Army High Command, was with the leading tanks near Bergues, and could look down into Dunkirk. He sent radio messages, asking to be allowed to push on, but was rebuffed.
It is true that the tanks were in a commanding position, but they were low on fuel, and without infantry support. They were also within range of British naval guns in the channel. Even a Panzerkampfwagen IV (PzKpfw IV) would be outmatched by a 4.5 inch naval shell! The tanks withdrew as ordered.
Hitler's own experiences in the trenches of the First World War were almost certainly a factor. By 24 May the troops had been fighting continuously for nearly a fortnight. Hitler knew how exhausting that could be.
Also, it is certainly true that the ground around the Dunkirk pocket, with its network of canals, was not ideal for tanks. The infantry needed time to catch up. General Franz Halder wrote in his diary:
"The Führer is terribly nervous. Afraid to take any chances."
General Halder's diary is also the source of the claim that Goering had persuaded Hitler to allow his Luftwaffe to finish off the encircled troops. His diary entry for 24 May states:
Finishing off the encircled enemy army is to be left to Air Force!
Halder's diaries have been translated and digitised, with the relevant entries for 24 May 1940 in Volume IV.
General Paul von Kleist met Hitler on the airfield at Cambrai a few days afterwards. He is supposed to have remarked that a great opportunity had been lost at Dunkirk. Hitler apparently replied:
"That may be so. But I did not want to send the tanks into the Flanders marshes".
There was also the belief among the German High Command that the war was already effectively won. A handwritten note from Major-General Alfred Jodl, the deputy chief of Hitler's planning staff still survives. It is dated 28 May and was written at Führer Headquarters to the labour minister Robert Ley. It states:
"Most esteemed Labour Führer of the Reich! Everything that has happened since May 10 seems even to us, who had indestructible faith in our success, like a dream. In a few days four fifths of the English Expeditionary Army and a great part of the best mobile French troops will be destroyed or captured. The next blow is ready to strike, and we can execute it at a ratio of 2:1, which has hitherto never been granted to a German field commander..."
After the war, perhaps not surprisingly, German generals vociferously blamed Hitler for the British "miracle" at Dunkirk. Even von Rundstedt placed the whole debacle at Hitler’s feet. This has led to the many theories about why Hitler had "allowed" the BEF to escape:
These are, of course all utter nonsense and have been dismissed by all credible historians. Sadly, they still seem to be regularly trotted out by assorted Hitler apologists like David Irving, despite all the surviving evidence that should have condemned them to the dustbin of history years ago.
The truth is much simpler. Hitler didn't entirely trust his army commanders and was being cautious. He, together with his military commanders, believed that he had time to regroup his forces and attack with the combination of infantry, artillery, armour, and air power that had already brought the German army success in France. The details of that build-up are set out in General Halder's diaries.
Even after the "Stop Order" was issued to the army on 24 May, the Luftwaffe continued to attack the troops on the beach at Dunkirk. Whether this was to allow Goering's Luftwaffe the final "glory" of defeating the BEF remains just speculation.
We should also remember that on 24 May the surrender of France was not yet assured. Neither Hitler, nor his high command, were prepared to risk unnecessary losses (as they saw it) that might put the next phase of their operation in jeopardy.
It's easy to ask these questions after the fact, but a primary reason was that what we now know as the Miracle of Dunkirk was basically unthinkable.
It is easy to forget that in reaching the coast, and cutting the Allied line in two, the Germans had already won a great, practically unthinkable victory. Their victorious divisions, particularly armored units, were scattered, considered overextended, and needed time to be "realigned." One can say that the best use of these troops was to pile on, pursue the enemy into the ground, etc. but that would not have been "easy." It would have been a "brawl, the kind of fighting that the Germans disliked, with British naval gunfire taking part, as a commenter pointed out. If the Germans had managed to slaughter 300,000 troops in this way, they would probably have taken casualties of a significant fraction of this, say 75,000-100,000 men. There might even have been embarrassing losses of key units or commanders. A victory that cost the life of say, Heinz Guderian, might have been very "bittersweet," and had us asking why the Germans didn't "hold up."
Nor was it really the German way. They were winning, and planned to win, but in a more organized fashion, with armor, infantry, artillery and airpower in alignment (although that gave the enemy a chance to reorganize as well). And speaking of airpower, that was supposed to play a key role in 1) blocking retreat and 2) actual annihilation. Much to a lot of people's surprise, it did neither.
The original British hope was evacuate 45,000 men over two days. In fact, they evacuated 338,226 men over a period of eight days. This was due to the efforts not only of the regular navy, but of "little ships," civilian motor boats, pleasure craft, etc. In fact, the port was blocked and the big ships could not get close enough to shore to embark many soldiers, so the smaller ships did the actual ferrying of these men. A combined military-civilian effort involving a total of almost 1000 ships of this kind had never been seen in the history of warfare.
Another imponderable factor was the effect of the good weather during the evacuation as pointed out by a commenter. "Forecasts" were probably available to both sides beforehand, and the Germans probably thought that clear skies would help their bombers. It turned out to help the numerous ships far more.
Then, Hitler was hoping for an early cease-fire/peace with Great Britain, and would therefore rather "capture" 300,000 or so British soldiers than slaughter an equivalent number. Bombing them would have fallen under the "capture" strategy; during the same campaign, German bombers that had run out of bombs terrorized French soldiers just by remaining overhead.
Basically, the Germans thought that they could take their time, and minimize their losses and disorganization for the battle with the remaining French forces, while capturing the bulk of the British army. They could not envision not only all the trapped Britishers but almost half of the French in the pocket escaping. The conventional wisdom was that the British could rescue their highest officers, (as the Germans did from say North Africa), but the rest of the men, including most of the non-coms and junior officers, would be stranded.
That's one great mystery in history. Some say that Goering asked Hitler to let the Luftwaffe handle this problem. Some say that Hitler was afraid of his armies going too fast. Some say that Hitler was afraid of the might of the RAF.
One point is not to be forgotten though. The French army, although completely late in panzer and plane tactics, fought very hard around Dunkirk to defend the harbour as long as possible, with more than 100,000 were killed in action during the 6 weeks of the battle of France. The French air army also destroyed around 1000 Luftwaffe planes during this period; those missing 1000 Nazi planes were really missed during the Battle of Britain. But because of the armistice of shame in June 1940, those poor fighters were forgotten by history, especially in the English speaking world.
From all I have read over the decades, the Germans were doing their very best to wipe out the Brits at Dunkirk. And there's a body count to prove it.
Let us not forget that Hitler assumed the UK would join him as he perceived the UK being of a similar mindset to Germany. At the time of Dunkirk, he was still hopeful the UK would join him as an ally, not an enemy and this may have stayed his hand and provided the opportunity to save the troops.
Just an idea....
