How’s this for a Crusades concept?

As a student of history, Ridley Scott knows as well as anyone that the Crusades have been given the gauzy, soft-focus treatment by Hollywood. What turned out to be “one of the blackest chapters in the history of Christendom,” where high ideals were “besmirched by cruelty and greed,” as one historian has put it, has often been treated as a noble adventure with exciting opportunities for swordplay.

“The Crusades have been incredibly romanticized, starting with Richard the Lionheart and King John,” Scott told me the other day. “The 200 years of the Crusades is just full of chronically bad behavior.”

Since few directors rank higher than Scott in my personal pantheon, I was too polite to point out that his new 20th Century Fox film, “Kingdom of Heaven,” does some serious romanticizing itself. The $140-million film, which had a mediocre $19.6-million opening this weekend, follows the exploits of Balian of Ibelin, a blacksmith played by Orlando Bloom who unsuccessfully defends Jerusalem against the great Muslim warlord Saladin in 1187 during a lull between the second and third Crusades.

Perhaps “exploits” is too strong a word, since in Scott’s film, written by William Monahan, Balian is a curiously passive figure. He’s an idealist, and an ineffectual one, since he does little to prevent bloodthirsty Christian warlords from undoing what had been a lengthy peace in the region. The critics for the most part have not been kind, saying Bloom looks like “a page boy impersonating a warrior” and bemoaning a storytelling approach that seems most concerned with not offending any religious group. The saga ends with Bloom agreeing to a graceful surrender instead of valiantly fighting to the death. As the New Yorker’s Anthony Lane put it: “He comes across more as a fellow to whom things happen than as somebody who can wrest events to his will.”

Even his love interest, the comely Princess Sibylla, played by Eva Green, is alarmed by his passivity, cautioning him, “There will be a day when you wish you had done a little evil for a greater good.” That’s the line in the film that had the most resonance for me, and not only because it’s equally applicable today -- one can easily imagine Iraq war architect Paul Wolfowitz embracing the princess as a seductive poster girl for neoconservatism. But Sibylla’s warning set off cinematic alarm bells. It’s not that Scott’s hero is too weak. If the Crusades are a classic example of the complexities of moral fervor, Scott chose the wrong Crusade story altogether.

Regardless of how “Heaven” ultimately fares at the box office, it’s hard to imagine there being another major Crusades-era film anytime soon. This would be a great loss, because there is a great story to be told, one that is recounted with wonderful vigor by historian Jonathan Phillips in his new book, “The Fourth Crusade and the Sack of Constantinople.” It is a tale teeming with unlikely heroes, canny operators and nasty brutes, plus bloody battles and incessant political intrigue, not to mention fascinating military ingenuity. In other words, it would make a great movie.

Roughly a dozen years after the fall of Jerusalem portrayed in Scott’s film, Pope Innocent III issued a call for a new Crusade to retake the holy city. Having seen how previous Crusades were undone by arduous overland journeys, these Crusaders decided to travel by sea. In need of an armada of ships and supplies, they turn to the wily Doge Enrico Dandolo of Venice, then Europe’s leading seafaring power. A strikingly modern figure, he was so ingenious at brokering deals and exploiting the weaknesses of friend or foe that he’s best described as a cross between Henry Kissinger and Lew Wasserman.

And when it came to contractual brinksmanship, the doge, even though he was blind and pushing 90, was the equal of anyone in Fox’s business affairs department. In return for securing provisions and building enough ships to transport 33,500 soldiers and horses, the doge demanded 85,000 marks, double the salaries of the kings of France and England. When barely a third of the expected soldiers showed up, leaving the Crusaders unable to pay up, the doge decided to restructure the deal. Thus began a series of detours that led the Crusaders farther away from Jerusalem and deeper into spiritual quicksand.

Eager to take back control of Zara, a neighboring city that had allied itself with the king of Hungary, the doge told the Crusaders he would postpone the debt if they first conquered Zara, a fellow Christian city. Talk about doing a little evil for a greater good. Zara was quickly subdued, but the Crusaders were still short of money. Enter Prince Alexius, the exiled heir to the throne of Constantinople, whose father, the rightful emperor, had been deposed.

Alexius promised the Crusaders a huge sum of money if they would again postpone freeing Jerusalem, this time by making a side trip to Constantinople, where they would put him on the throne. Once again, this involved attacking a Christian city, but the Crusaders justified it by assuring the pope that Alexius would place the Greek Orthodox Church under the rule of Rome.

Constantinople was protected by a formidable wall to the west and water on every other side, and the Crusaders’ attack was the equivalent of a 13th century D-day, representing the largest amphibious invasion ever attempted in medieval Europe. Having the Venetians along proved invaluable. They built canvas-covered bridges atop their ships, providing a protected walkway for knights who could disembark, fully armed, via scaling ladders when the ships reached the city’s battlements. At the height of the battle, fearing his men were losing courage, the blind doge demanded that his sailors put him ashore so he could lead the attack.

The usurper fled, but unrest continued. Soon a new warlord came to power, vowing to crush the Crusaders. Before they began their ultimate siege of the city, the Crusaders came to agreement on a decidedly ungodly subject: splitting the loot. It was a true Hollywood-style profit participation deal. The Crusaders promised the doge 75% of the spoils until Venice got its promised 200,000 marks. After that, the Venetians would take a 50-50 split of further booty.

In battle, the Venetians’ precision seamanship again proved invaluable. With knights perched high atop their flying bridges, they managed to maneuver close enough to the city walls to land men on the Greek battlements. The first invader was butchered. The second fell to his knees, hiding under his shield, surrounded by Greeks. But when he stood up, unhurt, and drew his sword, the Greeks fled. In victory, the Crusaders were far less charitable than Saladin in “Kingdom of Heaven.” The Crusaders’ pent-up anger unleashed a wave of rape, bloodletting and plunder.

The greater good had evaporated. The Crusaders never made it to Jerusalem, an outcome that, if you like, could easily be compared to the modern-day war against Al Qaeda that somehow took us into Baghdad. It’s what makes the Fourth Crusade seem so resonant today -- it’s a tragedy of good faith undone by blind self-righteousness. When I asked Scott what he thought made “Kingdom of Heaven” relevant for today’s audiences, he said he wanted to show the heroism of a good man like Balian. “We need a few idealists,” he said. “Actually, we need a lot of them.”

I couldn’t agree more. But by focusing on a weak-kneed idealist, Scott not only leaves a hole at the center of his story but misses a chance to show how much the conflicts of the Crusades have in common with our noble causes of today. When it comes to drama, the Fourth Crusade, which illustrates all too well the grievous limits of idealism, tells us more about the world as it is, not as we wish it to be.