What was Hitler's reasoning behind the May 1940 order to stop his military short of actually taking Dunkirk?

My answer on Quora

There is a great deal of debate on this issue as it certainly provided the British Expeditionary Force (BEF) with the biggest let off in the war’s turbulent history.

To begin with the halt order itself was not issued directly by Hitler but came through Generals von Kluge and von Runstedt. It is believed that there was debate within the German High Command with both generals fearing an Allied counterattack (such as what happened at the Marne in 1914). There was also some suggestion that the German army was exhausted by the speed of its own advance and needed to rest.

Another general, von Brauschitsch had disagreed with the halt order and favored an immediate attack but Hitler appeared to have overruled him( von Brauschitsch was the Army’s Chief of Staff).

Some have suggested that Hitler was intending to give an olive branch to Churchill but an analysis by the German historian Karl Heinz Friedser makes the case that this was not the fact the reason and that Hitler was instead using the occasion to make it clear to the Army that he was indeed the Supreme Commander.

Of interest is that the attack did resume on the 26th of May 1940 (two days after the original halt order) but by this stage the success of Operation Dynamo (the mass evacuation from Dunkirk) had negated the strategic advantage of this later move.