BEIJING — Historians reckon that 1 million soldiers took part in the battle of Red Cliff, the bloody confrontation that gave birth to China’s Three Kingdoms. Never one to shy away from a challenge, Hong Kong’s helmer-in-chief John Woo is planning to make sure his $80 million take on this piece of Chinese history will be as epic as the original battle itself.

“Red Cliff” is the most expensive movie ever made in Asia, with funding coming entirely from independent producers in the region.

For the Asian film business, “Red Cliff” is grandly ambitious — the financing is groundbreaking, the budget immense, the length fearsome and the special effects tipped to be awesome.

It’s got some of the biggest names in Asian cinema, including Woo himself, who is marking his return to Chinese-language movies with the pic and has a lot riding on its success. It’s also four hours long in one version and has had to overcome some serious casting changes.

Based on a segment of the sprawling classic Chinese novel “Romance of the Three Kingdoms,” pic is set in the final days of the Han Dynasty, in the year 208, and covers the war that established the Three Kingdoms period, when China had three rulers.

Not only is “Red Cliff” significant in terms of pan-Asian funding and budget, it’s also enormously significant in terms of technical ambition: Its producers see it as a CGI-infused epic for young auds steeped in manga-inspired ultraviolence and raised on the computer-generated images of “Sin City” and the kind of intensity that “300” brought to the historical war genre. Numerous units have shot simultaneously on the pic, and with all this footage floating around, there is plenty of scope for ratcheting up the shock and awe in the editing suite.

It also carries a burden of expectation that could break the back of a smaller movie. The makers of “Red Cliff” are keen to distance themselves from a series of me-too martial arts epics — such as “Seven Swords” and “The Banquet” — that China has been producing for the past few years, and there is also another project based on the “Three Kingdoms” novel, helmed by Daniel Lee starring Andy Lau and Maggie Q, set for release in 2008.

“Things are going fine. We’ve still got about a month to shoot, and we expect to wrap end-October, maybe mid-November,” says producer and Woo’s partner in Lion Rock Entertainment Terence Chang.

Chang adds the budget was “south of $80 million” — indeed, Woo has played down the cost of the movie in the media. The funding came from four territories: China’s China Film, CMC Entertainment in Taiwan, Avex in Japan and South Korea’s Showbox.

With so much money in place, and with Asian stars seeing their profiles rise in world cinema (Ang Lee’s Chinese-language “Lust, Caution” just won the Golden Lion at Venice), it was perhaps inevitable there would be personnel problems on the set, but “Red Cliff” has had some pretty major switcheroos.

In March, Tony Leung dropped out of the film. At the time, he said he felt unable to commit to the six-month shoot the pic demanded and he was replaced by Takeshi Kaneshiro.

Then Chow Yun-fat, a close ally of Woo, dropped the bombshell that he was leaving.

The circumstances of Chow’s departure were never clarified — local Chinese media said it was because of unreasonable demands by the thesp and conditions that completion bond company CineFinance could not accept. Chow countered those reports, saying the same firm bonded him twice before with the same requirements.

The idea of a falling-out between Woo and Chow electrified the film industry here. Woo made Chow a legend — casting him as an icon of cool in the 1986 Hong Kong pic “A Better Tomorrow” and genre classic “Hard Boiled.”

Two days after Chow ankled, Leung was back in the lineup in a different role than what he originally had contracted for.

The rest of the stellar cast includes Zhang Fengyi, Chiling Lin, Chang Chen, Vicky Zhao and Hu Jun.

Against this background of upheaval, it’s probably no surprise that Woo claimed it’s been the toughest film he’s ever worked on.

” ‘Red Cliff’ is the movie I’ve spent the most energy on, prepared for the longest, and is the most tiring since I started making movies,” he told the Southern Daily newspaper.

It’s Woo’s first bash at a Chinese-language project after many years in Hollywood helming projects such as “Broken Arrow,” “Face/Off” and “Mission: Impossible 2.”

And add this twist to “Red Cliff’s” journey to the bigscreen: The screenplay by Woo Chan Khan, Guo Zheng and Sheng Heyu is for a four-hour film. For Asian territories, pic is to be split into two parts, with the first part in July 2008 and the second part slated for December. Audiences outside Asia will get a single movie, expected to clock in at a fairly hefty 2½ hours, with a probable December release.

Pic, which is repped in international territories by L.A.-based Summit Entertainment, was widely sold at the February edition of the European Film Market.

“We’ve already sold it to a lot of European territories, but we are holding back on North America because people have a wrong impression about Chinese films there — they think of ‘Hero’ and ‘Crouching Tiger, Hidden Dragon,’ so we want to show them the movie when it’s finished,” says Chang.

But industry insiders say a lot of money is being asked for the North American rights, and studios are waiting it out, wary of spending too much money following the less-than-healthy international performances of pics such as Chen Kaige’s “The Promise” and Zhang Yimou’s “Curse of the Golden Flower.”

There’s also the Chinese government to contend with, which wants the movie to be a success because it will showcase Chinese history ahead of the Olympics in Beijing in 2008, says Chang, who describes the movie as a cross between “Troy,” “300” and “The Perfect Storm”: “There’s a lot of CGI in there.”

Undoubtedly, CGI will be a key factor. Many productions in Asia have suffered from uneven special effects, but “Red Cliff” is taking no chances: Effects pioneer Craig Hayes, who has developed revolutionary computer-generated graphic techniques and worked on “RoboCop” and the “Matrix” franchise, is visual effects supervisor.

Hayes was brought in by the San Francisco-based vfx house the Orphanage, which has worked most recently on the “Pirates of the Caribbean” series, “Live Free or Die Hard,” “Superman Returns” and the upcoming “Iron Man,” but some of the simpler stuff might be given to someone else, possibly in the region.

Extremes of weather have also taken an epic toll on the shoot. Torrential rains washed away part of an outdoor set in northern China; then during the summer and even now, the heat was and is hellish.

“We’re very affected by the weather. The hot weather now is bad if you’re wearing heavy armour,” says Chang.

For Woo, the pic is partly a patriotic exercise and a return to his roots. One of the key reasons for making “Red Cliff” such a major production is that he wants mainland Chinese crews to learn from the experience.

Given the cost of the movie and the big Asian names filling the armor on set mean the buzz about the movie is building.

Early indications are positive — the biz is cautiously optimistic, and Chinese government good will is assured. The Chinese computer game company Beijing Perfect World Technology has already announced a new game to tie in with the movie, “The War of the Red Cliff,” promising that it will break ground in Chinese gaming by remaining faithful to the original history while offering dizzying graphics and a strong concept.

“Red Cliff’s” producers will be hoping their pic can also break Asia’s cinematic mold.