bullet in the head (1990)

AKA Die xue jie tou. Bloodshed in the Streets. Wild Bullet.
"You're pointing a gun on a good friend you've known for more than a decade!"
Three friends (including Tony Leung), happily growing up in the Hong Kong of the late 60's, are thrown into chaos when they exact retribution on a gang member when he tries to mug one of them. The gangster dies and the three try to escape the police and the triad by moving to Vietnam. Unfortunately they end up in greater trouble as Saigon is in the beginings of the termoil that is to become the Vietnam war. As they get more and more embroiled in the events of the day their friendships become tested.
Another classic gangster film from John Woo, this time with a social conscience. The Hong Kong Deer Hunter.