Lu Chuan said he originally only planed to wade through a river, but he ended up struggling in an unfathomable ocean.

"I had underestimated the capacity and sensitivity of that part of history," Lu said, a day before his movie, "City of Life and Death," was to be screened nationwide in China.

The movie, which has took him nearly four years and cost 11 million U.S. dollars to produce, is about the Nanjing massacre of Chinese civilians by the Imperial Japanese Army nearly 72 years ago.

To produce a movie on the subject, which remains a heartrending memory to many Chinese and a stumbling block in China-Japan relations, challenged the director's wisdom, let alone the fact that a dozen of movies with similar themes had already been made.

"All I wanted, in the beginning, was to shoot a movie telling the world how Chinese people resisted the invaders. I didn't expect it could be so difficult," said 38-year-old Lu, who had four-year college education in a Nanjing-based military academy before entering the movie industry.

The Nanjing massacre is seen by most Chinese as one of the darkest chapters in history, during which the Japanese army slaughtered nearly 300,000 Chinese civilians and disarmed soldiers following the occupation of Nanjing, then capital of the Republic of China, in December 1937.

The topic remains a sore point in China-Japan relations. Many Chinese believed that Japan has not fully recognized and sincerely apologized for its war atrocities, while some Japanese historians and government officials frequently claimed that the massacre has been exaggerated or even fabricated for the purposes of political propaganda.