The Critical Mistake Hitler Made In The Battle Of Dunkirk

The Western Front of World War I had been at a stalemate for four years and cost the nations involved millions of lives. In 1939, when Germany and the Soviet Union jointly invaded Poland, both sides feared that the Anglo-French pledge to aid their ally would lead to a repeat of this destruction. Instead Hitler's Wehrmacht completely defeated France in six weeks, accomplishing what the Kaiser had only dreamed of. 

The French, British, and German armies all had artillery and tanks of their own deployed during the Battle of France, meaning the Axis victory was not so much technological as it was doctrinal. A heavy amount of intuition and luck kept them from getting into protracted engagements as they broke through the Ardennes, encircling the Allied armies. Yet when the bulk of the British Expeditionary Force and remaining French forces were trapped at Dunkirk, Hitler held his forces back for two days in an unexpected saving grace.

Hitler's reason for withholding his troops is a contested subject

Hitler's decision to temporarily halt the advance of his army arguably saved thousands of Allied soldiers on the beach and nobody to this day is entirely sure why he did it (via Mental Floss). His generals protested the move precisely because it gave the British desperately needed time but Hitler overruled them. One theory asserts that Hitler attempted to use this as an olive branch toward the British, whom he initially wished would either join him or sign an armistice (via Historic UK). That is unlikely though since Hermann Göring's Luftwaffe was turned to in lieu of ground forces during this period. 

While the planes failed to do enough damage, it was not for a lack of trying as they claimed hundreds of lives on the beach (via History.com). A more reasonable explanation is that Hitler was concerned about over-extension. His armored units were in prime position to move on the British, and in theory, could have wiped them out. However, this was only possible because they had sped far ahead of the German infantry. If the advance had been carried out per the wish of Hitler's generals, then the vehicles may have been overrun by the British, especially as their crews were exhausted after the often drug-fueled push (via Time.com).