Whatever about a movie changing the world, sometimes it can change a life. Maybe that's the same thing. Just ask Spike Lee.

"D'you know a funny story?" he says.

He's sitting under a white canopy on the beach in Cannes. It's almost 20 years since he first came here with Do The Right Thing, an explosive slice of Brooklyn neighbourhood life shot on a sweltering summer's day that culminates with a racist murder and a riot.

Although it lost out to fellow newcomer Steven Soderbergh's sex, lies and videotape for the Palme D'Or – and disgracefully failed to get an Oscar nomination for best picture, although it was a contender for best screenplay – it marked a turning point in American cinema, opening the way for other black directors including John Singleton, Carl Franklin and Mario Van Peebles.

"Look, back then I was complaining to you about the treatment when the Palme D'Or was handed out," he says. "But you just got to move on."

Not that it's been easy. Despite Malcolm X and the box-office success of his thriller Inside Man, he was unable to get American backing for his $43m Italian-set World War II drama Miracle at St Anna, a true story dealing with black American soldiers trapped behind Nazi lines. "I was hoping that having my big success would make it easier to make this film, but that has not been the case," he says. "I was crushed. But I'm not here to talk about that. History will bear witness."

He's wearing a New York Knicks basketball cap – he never misses any of their games when he's back home. There's a SAVE BROOKLYN slogan stitched to his jeans. It's where he grew up, son of a jazz musician, and most of his films are filmed there – St Anna is his first time filming abroad. His tee-shirt is emblazoned with the word Obama. Clothing for Lee is a billboard for things that concern him.

"So here's the story," he says. "I have a summer house in Martha's Vineyard, an island off the coast of Massachusetts. When Barack Obama was running to become senator, my wife Tonya and I went to a fundraiser there. He came over to me with his wife. 'Spike,' he said, 'you're responsible for me and Michelle getting together.' 'So how was that?' 'Well, you know, I wanted to ask Michelle to a date and the first date I took her out to was Do The Right Thing.'"

Lee chuckles. "I'm sure glad she liked it," he says. "Who knows how history would have turned out if she hadn't."

Ever since, he's spoken out for Obama. When Obama's former pastor Jeremiah Wright invited God to "damn America," Lee warned him off, saying "the more you open your mouth, the more damage you do."

He's convinced the bruising primary struggle with Hillary Clinton for the Democratic nomination has strengthened Obama. "It's like sports. The teams that are going to win are the teams that go through the fire, that have to fight all the way. The teams that lose – like the New England Patriots – glide through the season without being tested. Obama has come out hardened. He's been through the fire, and it makes you tougher.

"I know many people wanted Hillary, but when it all calms down they'll get their minds right. The intelligence of Americans could be questioned, sometimes. But after eight years of Bush, they'll do the right thing. It's a new day, finally."

He doesn't give the 71-year-old Republican John McCain much chance of winning. "I'm not imagining that," he says. "I'm going to be positive. I'm going to do all I can to make sure that does not happen."

Meanwhile, he has been squaring up to Clint Eastwood for not having any black soldiers in his two World War II movies Flags Of Our Fathers and Letters From Iwo Jima. "It seems extraordinary that the contribution of so many people has been airbrushed out of World War II movies," he says. "One million African Americans male and female fought for America but it's like they were never there. Our blood was never shed, our lives were never lost."

Eastwood responded curtly, saying "a guy like him should shut his face". Back in 1988, Lee had attacked him over his Charlie Parker biopic Bird, objecting to the idea of a white director making a movie about a black musician. "I was the only guy who made it, that's why," Eastwood pointed out.

"Clint sounds like an angry old man," says Lee, arguing that there's nothing personal, he's attacking Hollywood in general. "Woody Allen did all these films about Manhattan and everybody in them was white and living in the Upper East Side. Did anyone ever question him about that, or Steven Spielberg about Saving Private Ryan? So I'm not accusing anybody. I'm not jumping on anybody. I cannot speak for Steven Spielberg or Clint Eastwood or Woody Allen. But it's why I'm making Miracle At St Anna.

"The American army was segregated in 1944. The majority of black soldiers had white Southern officers. It was believed they knew them better. So right away it was a recipe for disaster. As well as contending with the Nazis they had to contend with racist commanders. At a time they were considered second-class citizens in America, they were fighting the Fascists in Italy. World War II was the ultimate case of good versus evil. It was about which way is the world going to go, democracy or fascism."

Early footage suggests that Lee has not shirked from depicting the full horrors of the Allied invasion of Italy. "But I'd like to stress that the black soldiers are only one element of the film," he says. "Religion, spirituality and faith are a crucial part of it. George Bush has made the good book bad, but ask anybody who has actually experienced war and they'll tell you how you come to an understanding of God.

"This film is the true story of four black soldiers trapped behind Nazi lines after one of them risks his life to save an Italian boy. I'm a fan of Italian neo-realism. I would like it to be seen in the tradition of Bicycle Thieves, Rome Open City, Obsession and Paisan."

He hopes to have it ready for either the Toronto or Venice Film Festival. "We've practically done the editing, and in a week or two we'll record the score with a 100-piece orchestra in LA. We expect a final print by the end of July and we're looking to a 10 October release Stateside." Although he acted in most of his early movies – he's a main character in Do The Right Thing – he won't be appearing in Miracle At St Anna. "I haven't been in one of my films for years. I never liked acting."

Now 51, Lee is unique in his willingness to confront unpalatable truths of American race and gender politics. Because of this and the anger that fuels his compassion – particularly notable in his expose of Hurricane Katrina in his HBO film When The Levees Broke – his public persona has been described as "intense, arrogant, driven and flagrantly talented."

"The whole thing that Spike Lee is an angry black man who walks around in a state of rage for 24 hours a day was old from day one," he says. "I don't know anyone who walks around constantly in a state of rage. You look at the body of my work. There's a lot of humour there. It has to come from somewhere."

Then I point to his SAVE BROOKLYN knee-patch. So what does Brooklyn need to be saved from?

"The Evil Empire," he beams.