Even if you haven’t seen Christopher Nolan’s new movie Dunkirk, you probably already know two things from all the glowing reviews: it’s intense, and it’s loud. Nolan has said time and again that this is not really a conventional war movie as much as a work of visual poetry—a white-knuckle thriller set during one of the greatest military rescues in history. There are, in fact, so many explosions, gunfire, and bombs in the movie that one of its stars, Northern Irish actor Kenneth Branagh, says that when he spoke to actual World War II veterans who had seen the film, they said it was louder than the real thing.

Branagh appeared on The Late Show with Stephen Colbert to talk about the new movie, which opens this weekend, and the two of them began by setting up some context. Colbert projected a map of the area while Branagh explained that the event inspired a kind of “miracle,” in which civilian boats from the southern shore of England, only 26 miles away, sailed across the channel to bring their boys back home.

Branagh said that about 30 veterans—now in their 90s—came to the U.K. premiere of the film last week, and when he asked them what they thought of it afterward, they said, “The film was louder than the battle!” They explained that on the actual beach, because that stretch of land is so large, the noise of the bombs drifted away on the air. But in Nolan’s film you hear everything.

Branagh also explained why Dunkirk in particular is so important to British citizens, who grow up with a “Never say die, never surrender” kind of spirit born out of this rescue. “The expectation, the hope from Winston Churchill, who’d been prime minister for just 16 days at that point, was that maybe they’d get 30,000 men back,” Branagh said. “That’s what they hoped for. And in the end, 360,000 people were rescued from that beach.”