The Peacemaker

In an Eastern Orthodox church in Pale, Bosnia and Herzegovina, an unidentified man (later revealed to be a Bosnian minister) is murdered after being paged to meet someone outside. (The murderer called him "minister").
At a missile base in Chelyabinsk, Russia, ten SS-18 ICBMs are being decommissioned. Ten nuclear warheads are loaded onto a train and sent to a separate site for dismantling. However, a corrupt high-ranking Russian general, Aleksandr Kodoroff, has other plans. Along with a rogue tactical unit, he kills all the other soldiers on board the transport train and then transfers nine of the warheads to another train. Kodoroff then activates the timer on the remaining warhead and sends the transport on a collision course with a passenger train. Minutes later, the 500-kiloton warhead detonates, killing the survivors and delaying an investigation.
The detonation immediately attracts the attention of the U.S. government. White House nuclear expert Dr. Julia Kelly (Kidman) believes that Chechen terrorists are behind the incident. U.S. Army Special Forces Lieutenant Colonel Thomas Devoe (Clooney) interrupts her briefing to suggest that the crash and detonation were staged to hide the hijacking of the other warheads on board. A call to Devoe's long-time friend and Russian counterpart, Dimitri Vertikoff (Mueller-Stahl), adds credence to his hypothesis and he is assigned as Dr. Kelly's military liaison.
Kelly and Devoe try to track the terrorists through an Austrian trucking company which is a front for the Russian Mafia. When the Mafia realizes they are U.S. government agents, they send thugs to kill them. Vertikoff, thinking he can pay them off, is soon killed. The Mafia then chase Devoe. He is able to outwit them with his superior driving skills. He kills most of the assassins, and he and Kelly escape. Information from the trucking company shows that the nukes are bound for Iran. Spy satellites place the truck in a traffic jam in Dagestan, Russia, and Devoe uses a ruse to identify it. The satellite, tracking in real time, is able to verify its license plate.
Stopped at a checkpoint, Kodoroff and his men kill the guards. Devoe then leads a special forces unit to stop them. Denied entry into Russian airspace, one of the helicopters is shot down by a Russian surface-to-air missile battery, but the remaining helicopters are able to locate the truck carrying the warheads. A gunfight ensues in which Kodoroff is killed and the warheads are seized. Interrogation of the surviving member of the group reveals that one warhead was taken by another man.
Further work on the information from the trucking company leads IFOR to a Sarajevo address. Inside is a video cassette of a Yugoslav named DuÅ¡an GavriÄ (IureÅ). GavriÄ disclaims any allegiance in the Yugoslav Wars ("I am a Serb, a Croat, and a Muslim"), but blames other countries for supplying weapons to all sides in the war. Dr. Kelly realizes he intends to bomb a meeting at the UN headquarters in New York City and the city goes into lockdown. GavriÄ arrives in Manhattan with the Bosnian diplomatic delegation. A flashback shows that GavriÄ wants to avenge the death of his wife and daughter, who were killed in Sarajevo's Sniper Alley. He and his brother are finally found by the New York Police Department. When his brother is killed by Devoe, a wounded GavriÄ is followed into a parochial school and then a small church. Devoe confronts GavriÄ, who commits suicide, knowing that the bomb is set to go off in a matter of minutes and cannot be deactivated. The authorities evacuate the people in the surrounding area. With only seconds to spare, Dr. Kelly is able to remove a part of the explosive lens shell of the bomb, preventing the primary explosion from establishing critical mass within the plutonium core. The primary wrecks the church, but the warhead itself does not detonate. Devoe and Kelly both survive with minor injuries.

Question: How is the helicopter shot down?
Answer: By a Russian surface-to-air missile battery.