Fifteen Man Squad

"There were fifteen of us, all boys from the village. But one of us betrayed the group. It was a setup... for money! Twelve people died, three escaped."

- Niko Bellic.

The "fifteen man squad" was a military squad which saw active service during the Yugoslav Wars. 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.

Background
According to Darko, the squad also killed civilians (Niko does not comment on this), some of whom were Darko's neighbours, which was his motivation to betray them.

Sometime during the war, the squad was ambushed by enemy forces. Twelve members of the squad were killed, with Niko, Florian Cravic and Darko Brevic surviving and escaping. Niko 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 found a large pit containing the squad members' bodies. Niko identified each of the bodies, but did not find those of Florian or Darko.

Concluding that one of the two had been the traitor within the squad, Niko vowed to track down and kill the culprit. Unbeknownst to Niko, Florian would later move to Liberty City, while Darko fled to Bucharest, Romania.

Events of GTA IV
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 in a relationship with Liberty City's deputy mayor, Bryce Dawkins. He protested his innocence, and Niko was angered to know that he would not get his revenge any time soon as he had no idea where Darko Brevic could be.

Later in the game, an agent of the United Liberty Paper Company located Darko in Bucharest. Darko was kidnapped and smuggled into Liberty City. Niko confronted Darko at the cargo bay of Francis International Airport 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. 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.

Conclusion
If the player chooses to kill Darko, Niko will shoot him twelve 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 that he is "proud" of him, and that both men now have the closure they desire.

If the player decides to spare Darko by walking away, but then manages to kill him while he is fleeing, the game will act as if Niko had chosen to kill him, but the cutscene will not play.

Trivia

 * In a conversation with Dwayne Forge during a friendship activity, Niko reveals that eight of the twelve squad members who died were friends of Niko, with the other four merely being acquaintances from the village.