While Don't Cry, Nanking is sometimes a propaganda vehicle for the Chinese, at the expense of the Japanese, it is a far more realistic portrayal of war than many Hollywood movies, including Saving Private Ryan.
The movie deals with the situation during the Japanese invasion of China prior to World War II, focusing on the massacre of hundreds of thousands of Chinese soldiers and civilians when Japan occupied the capital city of Nanking in 1937.
The story involves a couple, one of whom is a Chinese doctor, and the other is his Japanese wife, who have to deal with the invading soldiers. The doctor, being Chinese, has reason to worry about the invaders. The wife doesn't have to deal with the Japanese soldiers, but she is isolated among all the Chinese.
The reason the movie is realistic is because it details a relationship between parties from opposite sides of the conflict. Movies that show such relationships (and it doesn't have to be romantic) are more realistic than movies that don't, since there are always two sides to every story. The movie illustrates the cost of war to people on both sides of a given conflict. Also realistic is the fact that the actual decisions by the Japanese general who becomes responsible for the death of thousands are also justified in practical terms, not in any sadistic sense (though the individual soldiers' actions vary in cruelty).
The film illustrates the nature of violent conflict and is worth watching purely for comparison to any of the world's hot spots of violence today. The unequivocal truth is that not a single lesson has been learnt from these horrific conflicts and acts. I don't doubt for a single moment that the Chinese would have behaved just as ruthlessly as the Japanese were the roles reversed (especially given their record after 1937, considering that it was this set of events that led to the rise of Mao Tse-tung and the Communist government, and the China/Taiwan separation).
In my view, a lot of the organised violence occurs in this world purely because of the fondness of humans to ally themselves with monolithic institutions in the form of religion and government. Everyone is either being loyal or following orders or fighting for a cause. This allows one to easily disawow individual responsibility. It is far more likely that two populations making collective judgements would commit gross and systematic acts of violence, than would individual members of the same populations making isolated judgements, against each other. As long as people are not taught that individuality should be more valued than anything else and that each action performed requires careful thought, this collective sheep-like behaviour will only lead to further destruction.