NATO bombers have already attacked bridges, television stations, and the residence of Yugoslav President Slobodan Milosevic.
But if they want to defeat the Serbs through airstrikes or ground forces they will likely have to venture further along the gray line between civilian and military targets.
Serbian military strategy, analysts say, is far different from that of most NATO nations, where the focus is on preventing the enemy from entering the territory. The Serbs, rather, use a system based on a "Doctrine of Armed Citizens."
Although most of the doctrine does not officially exist under the 1992 Yugoslav Constitution, it has been taken up ad hoc. Under this doctrine, military defense is built for the long haul, designed for guerrilla resistance and the formation of light infantry units that operate in rugged terrain.
"We have the experience and the means," says retired Gen. Stevan Mirkovic, a former top commander of the Yugoslav Army. "The only way that one small nation can confront a big one is through this tactic." Heavy units of tanks, sometimes positioned at the borders, are meant not to stop invaders, but to delay them long enough that infantry units can fall back to the hills.
Also, as part of the doctrine, capitulation is not allowed, casting doubt on the possibility that Milosevic would give in to the demands of NATO and willingly allow troops in Kosovo.
Serbs now of fighting age - both male and female - were taught in high school how to use bombs. They learned how to fire rifles and how to disarm land mines.
Essentially, they learned how to be Partisans - the guerrillas of Josip Broz Tito who mounted a successful resistance to the 1941-45 Nazi occupation.
"It would be better to fight face-to-face," says Dusan, a cab driver in his 20s. "These slow airstrikes are like torture. At least if there is a ground invasion I'd have the chance to pick up a gun."
A strategy already in use
The effects of this strategy have already become evident after more than a month of NATO airstrikes, in which the Serbs have frustrated NATO with their unconventional tactics.
NATO has been foiled in attempts to thoroughly destroy oil-storage depots, to destroy heavy weaponry, and knock out the vaunted Yugoslav air-defense system.
NATO has found that massive stationary defense units almost do not exist. Rather, armament is dispersed in the woods, or even hidden underground.
Despite what NATO officials say, most men of fighting age have not been mobilized in Serbia. Rather, they are sitting at home, many boiling in rage, waiting until they are given a gun.
Already Kalashnikov machine guns have been distributed in southern Serbia near the border with Kosovo.
Furthermore, the northern region of Vojvodina, bordering Hungary - from which NATO would likely have to begin if they decided to launch a ground invasion - may only be a red herring.
While NATO has bombed there heavily, the Serbs are likely to be willing to abandon it because it is too flat to defend.
A final element of the principle is being played out even beyond Yugoslavia's borders, with ethnic Serbs organizing abroad and protesting the NATO attacks.
Roots in 1800s
The Doctrine of Armed Citizens dates back further that just World War II. It was developed in the mid-1800s, as centuries of Ottoman Turkish rule began to break down.
Rather than form an organized army, the Serbs, known as Hajduci, took to the hills for hit-and-run guerrilla warfare.
The strategy was also successfully used in World War I, when a superior alliance of Austria, Germany, and Bulgaria drove the regular Serbian Army to Greece. The only armed resisters left were small groups of guerrillas known as Chetniks.
The Chetniks, though small in number, frustrated the invaders and maintained morale until the regular Army could regroup and launch a successful offensive.
In World War II, armed Yugoslav citizens thwarted the Germans so much that the Nazis began killing 100 Serbs for each Nazi soldier killed, and 50 Serbs for each wounded.
There are already signs that Serbs in Belgrade may be willing to engage in a similar defense.
Airstrikes are becoming increasingly personal, with the lists of civilian deaths growing daily, and a scarcity of resources taking a toll on day-to-day life.
"I lost my godfather today," says Dejan, a journalist here, mourning the NATO attack on the Serbian state television headquarters over the weekend.
"This has become too much to bear," he says.
Residents of Belgrade almost unanimously say they can't sleep at night, with bomb sirens splitting the air and detonations shaking the windows.
Many salaries have been reduced to about $30 a month, enough for a week's food. TV broadcasts, often the only source of entertainment, have been bombed off the air.
"God help us," says one mother of two children. "Life has been hard for 10 years. Now I don't know how to feed my children."