When the ship carrying Tommy, Alex and Gibson is sunk, Gibson saves the other two by opening the hatch when he could have easily swam for his life instead. The fact that he's shown visibly hesitating makes it even more courageous, as well as him being French, not even British, defying Cheese-Eating Surrender Monkeys.
The dogfights between British Spitfires and German Messerschmitt Bf 109s are breathtaking:
Farrier is an Ace Pilot, and demonstrates it several times with his Supermarine Spitfire, taking down several German planes on his own. He also very much performs a Heroic Sacrifice, staying much longer over Dunkirk and its coast than his plane's fuel reserve allows him in order to protect the embarking army and the boats coming for them, not expecting to come back home with it. In the end, the only solution he has is to glide and land, which he does perfectly, but it's in now German-controlled territory... His wingman, Collins, is no slouch either, shooting down a plane before being forced to ditch, and when rescued by the crew of the Moonstone, he immediately pitches in and helps with the evacuation.
Farrier's final act in flight is to shoot down an attacking German Stuka after his plane has run out of fuel, causing the whole beach to erupt in a massive cheer.
The faceless German tail gunner of the He-111 bomber attacking the minesweeper is so skilled with his weapon—a single machine gun against the eight of a Spitfire—that he actually comes the closest out of any other German in the film to killing Farrier, who had been gunning down numerous much more maneuverable and powerful Bf-109 fighter aircraft up until that point. In fact, he manages to shoot straight into Farrier's cockpit several times before he is finally killed along with the rest of his crew only after a solid six-second duel where he and the Spitfire pilot fire continuously at each other.
After all the devastation and terror of the desperate evacuation, Bolton and Winnant look out to the sea and spot numerous silhouettes on the horizon. Cue a sweeping shot of dozens of sailboats, fishing vessels and yachts, manned by civilian sailors with resolute expressions in their faces, moving to the beach to help the stranded soldiers get home as the score bursts into an orchestral swell that marks the movie's first unambiguously triumphant moment. Even better? Quite a good number of of those "Little Ships" actually partook in the real Dunkirk evacuation.
Mr. Dawson's small, unarmed, overloaded boat is spotted by a German fighter. Dawson never panics. Instead, he calmly gives the orders for evasive action using his knowledge of fighter tactics—and it works.
Everything about Farrier's landing on the beach. He calmly brings the plane to a perfect landing, removes his flight gear, sets his plane on fire, and stoically waits to be captured. You can almost feel the silent "fuck you" in the final shot.
During an early Stuka attack, a prominent British soldier displays sheer guts firing on the attacking aircraft repeatedly with nothing more than his Lee-Enfield bolt-action rifle.
Peter, a civilian teenager with no prior military service or combat experience, saves the life of an older RAF pilot through his quick-thinking; he uses a boat hook stored aboard the Moonstone as a fire axe in order to free Collins when the latter is trapped inside his sinking Spitfire's jammed cockpit.
Ken Sturdy, a 97 year old Royal Navy veteran who took part in the evacuation, gave tremendous praise to the film's accuracy and was moved to tears by the people who went through the battle now having this tribute.
On the technical side of things, the Air segments are a crowning moment of cinematographic achievement. Every last frame is a work of pure art. The shooting process involved strapping IMAX cameras onto an aircraft modified to resemble a Spitfire as to create a sense of scale and raw physicality that simply isn't possible with CGI. On one occasion, the scene called for a replica Spitfire to be crashed into the water, and a failure of the waterproof housing resulted in the camera being completely submerged. Nolan's crew contacted a film lab and managed to preserve the film by using an old technique that entailed keeping it wet as it was shipped to the studio in Los Angeles. The result was one of the movie's most memorable shots.
The simple fact that Nolan managed to pull off being incredibly intense war movie with only a PG-13 rating, something that many doubted could be done.