I'm providing links to previous discussions just for convenience and fast, reasonably authoritative answers. This is not to discourage further questions and debate -- please feel free to raise any other points that you like to.

In honor of the movie Dunkirk, there was a megathread some 6 months ago: "DUNKIRK MEGATHREAD: The Battle of Dunkirk and the Fall of France" at https://www.reddit.com/r/AskHistorians/comments/6pg8vd/dunkirk_megathread_the_battle_of_dunkirk_and_the/

It has a link to "Why did Hitler not crush the British army when he had it surrounded and cut off at Dunkirk?", with an answer by /u/threechance (and a lot of others) under https://www.reddit.com/r/AskHistorians/comments/6lds2b/why_did_hitler_not_crush_the_british_army_when_he/djtaq0b/

But it also links to a lot of other discussions, which you might find interesting if you are curious about other aspects.

You might triangulate with "why dident the Nazis fight the 400,000 stranded Brits in Dunkirk?", with /u/vonadler's answer, at https://www.reddit.com/r/AskHistorians/comments/6kfmhk/why_dident_the_nazis_fight_the_400000_stranded/