Until now, two weeks of NATO strikes have been characterized by pinpoint accuracy, with allied pilots striking military targets and other strategic points like bridges and oil refineries.
But the night of April 5, two residential neighborhoods in this poor mining city were devastated, killing about 10 Serbian civilians. The mishap could raise a red flag in the NATO report card and could galvanize a growing movement in the United States and Europe against bombing in the name of peace.
On the morning after the assault, rescue workers sifted through the debris, while survivors looked on in horror. It appeared as if at least two bombs had gone astray.
Yugoslav officials have claimed all along that NATO has been hitting civilian targets, but this was the first such site to be confirmed by Western journalists, who were given access to the site.
"It is possible that one of our weapons fell short of the target," said British Air Commodore David Wilby, a NATO spokesman in Brussels, who went on to speculate that there had been a "technical defect" in the bomb or a "guidance fault."
As the mission widens in scope, the allies are likely to undertake more difficult assignments and pose a greater risk to civilians. Improved weather allowed the alliance to attack nearly 30 targets on the night of April 5.
It's all likely to make the Serbs even more defiant - and less likely to sign a NATO-enforced peace agreement to give Kosovo autonomy.
The Yugoslav government, however, declared a unilateral cease-fire to mark Orthodox Easter, which is April 11. But Western officials replied the declaration would not end the NATO air campaign.
Still, on April 6, Yugoslav President Slobodan Milosevic showed defiance. "Those people who bombed Aleksinac can't be compared with humans," he reportedly said to Greek officials visiting Belgrade.
"[NATO] is not brave enough to fight us face to face," said Olga, a Serbian woman who witnessed the destruction in Aleksinac. "I would not regret if I died as a soldier, but I would regret it if I died like a dog, like these people did."
Aleksinac, where a military barracks on the outskirts of town was also reportedly hit, has a population of almost 30,000.
A woman living just 30 yards from one point of impact took reporters through the remains of her house, which was scattered with broken glass, clothes, and bits of furniture that were hardly recognizable. About a dozen eggs were untouched on the kitchen table.
"We went out on the street when we heard the airplanes," says Kosovka Siminovic. "I called out to my mother then the bomb came. Everyone was screaming. I crouched under the door frame. I don't remember anything else."
In the ruins of a house nearby, body parts could be seen sticking out of a massive pile of bricks and twisted metal that was littered with plastic decorative flowers, old shoes, and a Rubik's Cube.
Next door was a health center that also had been severely damaged. According to a witness, there were 10 people inside who were evacuated to another hospital.
"The Nazis weren't this bad," said Ms. Siminovic, making reference to the German Luftwaffe bombing of Belgrade, on this day 58 years ago. "They were good in comparison," said Siminovic, who was 13 at the time of the earlier bombing.
At the site where another bomb landed, in a wealthier part of town, at least 15 houses had been destroyed, and neighbors said three people were killed.
"These are ordinary people," said Lidija Petrovic, a neighbor. "It's terrible. My little son is still in shock."
Another neighbor, Vladimir Stojakovic, an electrical engineer, told how one woman had saved two children by pulling them out of the rubble and taking them to a hospital. When she returned, she discovered that she was too late to save her other relatives, who were buried beneath the rubble.
"I lost two houses," said Mr. Stojakovic, who "left the place an hour before the attack. These are my neighbors. We built our houses at the same time."
When describing one of the victims, a pensioner named Dragan Miladinovic, Stojakovic said: "He was poor, but hard working."