Serbian soldiers are very good guerilla fighters when unified and fighting for a common cause. If US sent ground troops into Serbia proper during the Kosovo campaign, even Vietnam would look like paradise.
There has been a lot of talk by the traitorus Boris Tadic to make the military service voluntary again rather than mandatory. In my opinion, that is a very very bad idea. Mandatory military service was first introduced after WW I because Serbs lost almost 1/4 of their male population because they were unprepared for a modern war. However, if you have a large army reserve, the chance of another repeat of 1914-1918 are much smaller.