Sergeant Nicolo "Nico" Toscani (Steven Seagal), a native of Palermo, Italy, is a detective in the Chicago Police Departmentâs vice squad. At an early age he became interested in martial arts, and moved to Japan to study.
In 1969, Nico was recruited into the CIA by Special Agent Nelson Fox and was involved in covert operations on the Vietnamese-Cambodian border during the Vietnam War in 1973. There, he became disgusted with Kurt Zagon, who tortured prisoners. A stand-off occurred when Nico tried to stop a torture session, and he left the CIA. Nico returned to Chicago, joined the CPD, and got married.
Nico and his new partner Detective Delores "Jacks" Jackson (Pam Grier), are now investigating a drug ring, and after busting two of the dealers, including Salvadorian drug dealer Tony Salvano, Nico finds C4 explosives. Shortly afterward, the men that Nico and Jacks arrested are released at the request of Federal officials, and Nico is asked to stand down. Later, the priest of Nicoâs parish is killed in an explosion during Mass. Fox calls Nico and tells him to move his family to a safer location, saying that Nico is in danger.
Under pressure from the Feds, Nico is asked to turn in his badge. Nico eventually finds that the dealers he busted are linked to Zagon, who is still with the CIA, and who is accused of human rights violations by a Central American priest who was being sheltered by Nico's priest. While Zagon is torturing the priest, Nico bursts in and a gun battle ensues. Detective Lukich (Ron Dean) and Jacks are wounded during the shoot-out, and Nico has to flee.
Senator Ernest Harrison is investigating Zagon's group to reveal their covert operations and drug dealing. When Nico finds out that Zagon killed the priest and is planning to kill Harrison, he goes after Zagon. Nico confronts Fox, but they are interrupted by Zagon's men. Fox is killed and Nico is captured.
Nico is held in the kitchen of a hotel during a Harrison campaign rally. Before Zagon can kill Harrison, Nico breaks free and kills Zagon and all of his remaining men. After, Nico meets Sen. Harrison, who has been informed of everything. Harrison promises justice for what they did and Nico says he is now willing to testify on his experiences with Zagon and covert operations in the CIA.
He also warns about keeping CIA and people like them in check in order to prevent them from thinking they are above the law, the root of ZagonÂ´s behaviour.

Question: What happens to Nico during the Vietnam War?
Answer: Nico stops a torture session led by Kurt Zagon.
Alternative answer: he left the CIA