Why was the retreat at Dunkirk critical?

Because the evacuation from Dunkirk saved 338,226 soldiers [British & French]. Although a massive amount of equipment was lost, Dunkirk was probably Hitler's greatest mistake next to attacking Russia because if he had pressed home his attack he would have as good as wiped-out the army and destroyed moral. The saving of so many soldiers, against all the odds, gave a massive boost to moral in Britain - a boost that was never to be lowered, even during the blitz.