During the course of the game, Niko talks about his experiences in the army, but what army is he speaking of? Why did he have to join?
1 Answer
Niko hails from what is now Serbia, which was once part of Yugoslavia in Eastern Europe. During the 1990s, this region was in a state of frequent turmoil and civil war. It's heavily implied that Niko was coerced or forced into service in the army as a teen. The experience scarred him, and he deals with the fallout from his (and his friend's/family's) actions during the war over the course of the game.
Further discussion of the Yugoslav Wars is probably outside the scope of this site, but there's a good Wikipedia article on it.