The success of D-day, a pivotal moment in World War II, partially hinged on the weather forecast. The Allied invasion of Normandy, France, on June 6, 1944, was planned for months as the American and British forces held practice operations in England.

Enormous efforts were made to mislead the Germans about what was coming. The operation was originally scheduled for June 5 but the day before, James Stagg, a meteorologist and group captain in the Royal Air Force, advised the American commander, Dwight D. Eisenhower, to wait for better conditions.

This lesser-known decision is the premise of “Pressure,” a new movie from filmmaker Anthony Maras. It’s an adaptation of David Haig’s play of the same name, in which the playwright himself portrayed Stagg. Haig, who co-wrote the “Pressure” screenplay with Maras, compares it to “The Imitation Game.”

“Some of these heroes who affect history from the sidelines just stay in the sidelines until somebody does research, discovers them lurking and finds they are so quietly heroic that it’s irresistible as a story,” Haig says, speaking via Zoom from London.

Haig began writing a version of the script shortly after the play debuted at the Royal Lyceum Theatre in Edinburgh in May 2014. It moved to the West End in 2018, and opened in North America at Toronto’s Royal Alexandra Theatre in 2023. Maras came onboard after making his 2018 film “Hotel Mumbai,” also based on a true story.

“When I first read the play and the script, I was bowled over by how, with this one decision, so many lives were changed,” Maras says, on a video call from Los Angeles. “Not just the lives of the men on the beach but throughout the Allied world. When you think of a war story, you think of men and now women on the field, but there is so much more to it behind the scenes.”

The film expands Haig’s play and includes additional characters and sequences, including the actual D-day invasion. It stars Andrew Scott as Stagg, Brendan Fraser as Eisenhower, Kerry Condon as Eisenhower’s secretary Kay Summersby, Chris Messina as U.S. Air Force meteorologist Irving P. Krick and Damian Lewis as senior British army officer Bernard Montgomery.

Both Haig and Maras strove to be as historically accurate as possible, even including archival footage from the war. “It is inevitably heightened, as any stage play or film is,” Haig says. “But it is very true.”

“It is absolutely as true as we could get it within the confines of a two-hour runtime,” Maras adds. “We took great lengths to try and be as accurate to the history but also to the deeper story as possible.”

Here’s what is true and what is dramatized in “Pressure.”

The importance of the weather

D-day, secretly known as Operation Overlord, was timed based on several factors, including the weather, the tides and the moonlight. Because the assault was multipronged, with Allied forces coming by sea, land and air, they required good visibility at night and a high tide to ensure less distances between the boats and the defending Germans.

“There were hundreds of meters between low tide and high tide,” Maras says. “So depending on where the boats landed, you either had 50 meters until you made it to the dunes and then the bunkers, or you had to make it 300 meters if it was low tide.”

A clear forecast with low winds and no rain was essential.

“The landing craft were antiquated and flat-bottomed,” Haig says, “and if they had gone on May 5 with the storms that Stagg anticipated coming in with the jet stream, those landing craft would have capsized. The war wouldn’t have been lost, although we do posit that it might have been in the film. In reality, failure would have elongated [the war] and caused countless extra deaths.”

To shoot “Pressure,” the filmmakers used real charts and meteorological instruments. The production design team re-created the famous D-day map from the Allied headquarters in Southwark House. The real one was made in two pieces by separate manufacturers to ensure secrecy.

“When you see that map, it’s a little bit mismatched and our team re-created that,” Maras says. “We got the paper they used to draw the maps from the same mill they used for those maps 80 years ago. A lot of effort was put into the minutiae that adds to the accuracy.”

Exercise Tiger

The film opens with a depiction of an Allied training operation called Exercise Tiger, which took place over several months on England’s Slapton Sands. Because many of the soldiers were young and untested, the Allied leaders wanted to prepare them for the sights and sounds of battle.

“They did a whole series of exercises to try and get together a full-scale dress rehearsal of what D-day would be,” Maras says.

These rehearsals, still widely unknown and spanning from late 1943 through April 1944, involved dangerous friendly fire and suffered from serious coordination errors, resulting in the real-life deaths of at least 700 American and British soldiers.

“That was an absolute disaster and yet we remember D-day as one of the great military triumphs in history,” Haig says.

Maras wanted the film to begin with this moment to emphasize the headspace of the Allied leaders.

“How do you establish what the true consequences of failure are for a story like this?” Maras says. “When we’re in the war room with all of those commanders and officers, they know what the implications of their words mean because they’ve seen it. They’ve lived it. The image of the blood in the water and the young men in that water was to tattoo in the audience’s brain that if these commanders mess up, this could happen again.”

Eisenhower, in particular, felt the magnitude of D-day. “He wrote two letters on the eve of D-day: what happens in success and what happens in failure,” Maras says. “He was sleeping two hours a night. He was a nervous wreck.”

Stagg vs. Krick

In the film, Scott’s Stagg arrives at Southwark House from Dunstable four days before D-day is planned. He is confronted by the American meteorologist Krick, who disagrees with him about the potentially disastrous forecast. Krick believes sun and calm seas are on the horizon thanks to historical analogue charts, but Stagg, using more comprehensive prediction methods, thinks a major storm is coming.

“In actuality, Stagg came onboard in about November 1943 and got to Southwark House a few months earlier,” Maras says. “His transfer came a few months earlier, not a few days earlier. The contours of the relationships between Stagg and Krick and the others are accurate, but they took place in a more compressed timeline.”

Both Stagg and Krick have recounted their version of events in various books, both claiming they were right about the weather. Although Haig and Maras imagine their dialogue and how these conflicts may have played out, the conflicts were real.

“They both adhered to their own meteorological vision,” Haig says, explaining the differences in prediction models from continent to continent. “In the United States, Krick’s system of weather forecasting was viable. If you come to the U.K., you can’t rely on the weather for more than five minutes, so that method doesn’t apply.”

Adds Maras, “They thought, ‘The weather is going to be good. We should hold our nerve and go.’ There was a rhetorically violent disagreement between him and the others.”

In the film, Krick claims that he has never inaccurately predicted the weather ahead of a battle, using his successes in North Africa as evidence. This was technically true.

“He was very good at his job within the context of certain geographical landscapes,” Haig says. “He didn’t make a mistake in North Africa. When Eisenhower challenges Stagg, he says, ‘This man never got it wrong.’ And he didn’t. In the whole of the North African campaign, Krick was spot on.”

After Stagg convinces the leaders to postpone D-day, he is vindicated by a deluge of rain that arrives while everyone is attending church at Southwark House on June 5. There was a church on site, although this moment in the film was dramatized.

“Whether it began raining precisely at that moment I have my doubts,” Haig says. “But it has the framework of truth.”

Ike and Kay

Andrew Scott and Kerry Condon in the movie “Pressure.”

(Alex Bailey / Focus Features / StudioCanal)

Kay Summersby had been an ambulance driver during the Blitz. The film hints at a less-than-professional relationship between Eisenhower and his personal secretary. She was certainly with Eisenhower at Southwark House, although there is less evidence that she had any kind of association with Stagg.

“The biggest fictional thing I did with both the play and the film was to join the third point of the triangle so you’ve got Stagg, Eisenhower and Kay,” Haig says. “The link between Stagg and Kay historically would be tenuous.”

There are differing opinions about Eisenhower and Kay’s relationship. “We know that they were extremely close and they shared a trustful bond,” Maras says. “There are many photos of them together. She was definitely a big force in Ike’s life at that time, and we wanted to pay respect to that.”

“Whatever one’s interpretation of the relationships that she inhabits within the story, her influence was substantial,” Haig adds.

After seeing Peter Jackson’s 2018 World War I documentary “They Shall Not Grow Old,” Maras had the idea to use colorized archival footage in “Pressure.”

“In the D-day sequence at the end, there are various real-life shots of the soldiers landing on the beaches,” Maras says. “We were able to cut between the archival [material] and our footage to increase the scope. And it wasn’t just to get the scale. Yes, we have shots of massive flotillas and ships and trucks, but sometimes it was just for a glance of a soldier where you can see death in his eyes.”

The team ultimately acquired more than 50 hours of archival footage. They hired research editors to go through it and, after a few days, Maras asked if any of the editors could recommend additional crew to help.

Then a man named James Stagg showed up to work. “Stagg’s grandson, 80 years later, walked into our offices and helped edit the archival movie footage that we put in his grandfather’s film,” Maras says.

Stagg’s wife

Andrew Scott in the movie “Pressure.”

(Alex Bailey / Focus Features / StudioCanal)

The play doesn’t include scenes with Stagg’s wife, Elizabeth, but Haig purposefully bookends the film with the couple together. “When he arrives at Southwark House as a terse, brusque, tricky man, you’ve already experienced his level of affection with his wife and that’s really important contextually,” Haig says. “You’re waiting for the end when he goes back to see her and the baby.”

At the time when Stagg went to Southwark House, his wife was pregnant. Stagg was not allowed to make phone calls to her because of the secrecy surrounding D-day. In reality, the hospital where she gave birth was not bombed, as it is in the movie.

“The bombing of the hospital was more reflective of the times that Stagg and his wife had gone through in the lead up to D-day,” Maras says. “That element is to encapsulate that Stagg was fearing for his wife. As he walks down this corridor, he is faced with: Is she alive? Is she dead?”

Truth to power

Ultimately, Stagg tells a room full of military leaders that they have to pause on D-day because of the weather — a truthful inclusion. It was important to Maras to emphasize how he stood up to power.

“Here’s a protagonist who’s not afraid to speak his mind and has the courage to get up in front of a room full of the most powerful military on Earth at that point and tell them something they don’t want to hear,” Maras says.

“When Eisenhower was passing on the baton of leadership at the inauguration for JFK, JFK asked, ‘What gave you the edge on D-day?’ Eisenhower said, ‘We had better meteorologists than the Germans.’ He had the wisdom to trust in the experts. It’s worth heeding that lesson from history.”