It's actually a hard film to classify, since in addition to being a crime thriller, it's also a spy film in a pre-war setting and there are elements of human drama and romance that touch on honour, friendship, patriotism and respect. I don't know if that has affected its success among a public that all-too-often tends to pigeonhole the genres it wants to watch.
The story begins by following up a long and close friendship, but Cusack arrives too late to save his life. Seeking to avenge him, and also do his job as an undercover American agent, he plunges into the dark world of growing Japanese domination and Chinese resistence in an impressively convincing recreation of pre-war Shanghai (despite being forced to relocate to Bangkok a week before shooting).
This brings him into contact with some fascinating characters. There's the charming crime lord, played by Chow Yun Fat in a style reminiscent of Alan Delon, the beautiful but mysterious Gong Li who is so easy to fall in love with, and Ken Watanabe as the head of Japanese intelligence, who is efficient yet still manages to maintain romantic vestiges.
The story builds to an exciting and complex crescendo, as Japan attacks Pearl Harbour and takes over the Shanghai international quarter and the individual interests of the main characters come into conflict. It is well worth watching this film to see how it all plays out.