The result was the greatest and most important Maritime evacuation in military history as a fleet of British military and civilian ships of about 800 evacuated approximately 350,000 of those troops to England. Lose those troops and the Allies probably lose the war long before the United States of America joins it.
Christopher Nolan doesn't attempt a wide-reaching, expository historical epic here. Instead, he focuses his film on three targets operating on three different but converging timelines. That would be a week on the beach with the waiting troops, a day on a civilian-piloted boat helping with the evacuation, and an hour in the air with RAF pilots engaging the Nazi air force over sea and land.
It all works beautifully. The only drag is a fictional sub-plot on the boat that seems clumsy and obvious. Otherwise, Dunkirk is a war movie that portrays the fear and tension of warfare in a number of set-pieces. The only traditional war-movie 'release' comes with the RAF's battles with the German fighters and bombers. Otherwise, Dunkirk is the war movie as psychological horror, with groups of men listening for the sounds of bombs dropping through the air, pinned down under fire from an enemy they can't see, or struggling to escape the flooding compartments of a sinking rescue ship.
Civilian boat captain Mark Rylance and Commander-on-the-beach Kenneth Branagh supply the personable acting here, with smaller turns from several of the young actors who portray the troops and Tom Hardy as one of the RAF pilots. The aerial combat scenes are thrilling and expansive; the rescues at sea are thrilling and horrifying. It's a marvelous, focused movie (less than 2 hours long!), and it may come to be regarded as Christopher Nolan's best. Highly recommended.