The "fifteen man squad" was a squad (proper name unknown) of child soldiers during the Bosnian War. Members of this squad included Niko Bellic, Florian Cravic, Darko Brevic, Goran, Mijo, Dragan, Dmitar, and eight other unnamed members. All had grown up together in the same village.
According to Darko and not denied by Niko, the squad killed civilians of which some were Darko's neighbours which was his motivation to betray them. Considering the fact that they were Serbs, especially if they were Bosnian Serbs, this well might have happened.
Sometime during the war, the squad was ambushed by enemy forces; Niko escaped. He believed that the squad could not have been ambushed unless someone within the group had sold them out; he travelled back to where the ambush had taken place, and saw a large pit with the squad members dead inside. After picking through the bodies, Niko realised that two others were missing: Florian Cravic and Darko Brevic.
Concluding that one of the two had been the traitor within the squad, Niko vowed to track down and kill the culprit. Unknown to Niko, Florian would flee to Liberty City, and Darko to Bucharest, Romania.
Events of GTA IV
Roughly ten-to-thirteen years later in 2008, Niko arrived in Liberty City, both to visit his cousin Roman, and to track down Florian Cravic, whom he had learned was living there. Eventually, Niko tracked Florian down — who had come out as homosexual, and was having a fling with Liberty City's closeted deputy mayor, Bryce Dawkins. He protested his innocence, and Niko was angered to know that he would not get his revenge any time soon — he had no idea where Darko Brevic could be.
Later in the game, an agent of the United Liberty Paper Company located Darko as a favor to Niko, and had him kidnapped from Bucharest and left at Francis International Airport. Niko confronted Darko, and discovered that Darko had sold the squad out for $1,000 to fund his heroin addiction. Niko was all the more appalled that he and his friends were sold out over such a paltry sum; Darko called Niko a hypocrite, knowing that Niko had frequently killed for money too. He also told Niko that Goran had killed Darko's friends and neighbours, giving Darko another reason to betray the unit.
The decision to kill Darko or let him live is then left to the player.
If the player chooses to kill Darko, Niko will shoot him 12 times, one shot for each fallen squad member. Before dying, Darko will thank Niko. Roman, who is with Niko during the confrontation, says that it is over now and Niko says he feels empty for killing Darko.
If the player lets Darko live, he runs off, never to be seen again. Niko is angry with himself for letting Darko live, but eventually decides that it was the best choice, and feels better. He lets Florian know, and Florian tells Niko he's proud of him. Niko finally has closure on that part of his past.
If the player decides to spare Darko by walking away, but then manages to kill him while he is fleeing, the game will act just like if Niko had chosen to kill him, but won't play a cutscene.
- In a conversation with Dwayne Forge during an activity, Niko reveals that eight of the twelve squad members that died were friends of Niko, the other four merely being acquaintances from the village.
- If the player decides to execute Darko with a Desert eagle (which holds a capacity of 9 bullets) Niko will still shoot Darko 12 times without reloading.