“I WATCHED a little baby die today. Absolutely horrific. There is just shells, rockets and tank fire pouring into civilian areas of this city and it is just unrelenting.” Thus reported Marie Colvin, an American journalist of extraordinary courage and integrity, to the BBC on Tuesday from the Syrian city of Homs. On Wednesday she was dead, killed along with the French photographer Remi Ochlik, reportedly by rocket fire.

The death of Ms. Colvin, who covered wars around the world for Britain’s Sunday Times, was a tragic loss for Western journalism. It followed by days the death of another great reporter seeking to cover the slaughter in Syria, Anthony Shadid of the New York Times — a two-time Pulitzer Prize winner for The Post.

If they serve any purpose, these terrible blows to our profession should call attention to what Ms. Colvin was witnessing when she died: the “absolutely sickening” assault by Bashar al-Assad’s troops on the Homs neighborhood of Baba Amr — where an act of mass murder is underway. “The Syrian Army,” she told CNN in another interview Tuesday, “is simply shelling a city of cold, starving civilians.”

The siege of Baba Amr, which houses 28,000 people, began on Feb. 4. According to a riveting dispatch by Ms. Colvin published last weekend in the Sunday Times, she sneaked into the city on a smugglers’ route, “climbing over walls in the dark and slipping into muddy trenches.” She was working out of a makeshift news center in a building whose top floor, she reported, had already been blown off by artillery fire. The site may have been targeted by regime forces tracking satellite telephones; two other foreign journalists were reported wounded in the attack.

Here is how Ms. Colvin described Homs: “It is a city of the cold and hungry, echoing to exploding shells and bursts of gunfire. There are no telephones and the electricity has been cut off. Few homes have diesel for the tin stoves they rely on for heat in the coldest winter that anyone can remember.. . . The scale of human tragedy in the city is immense. The inhabitants are living in terror. Almost every family seems to have suffered the death or injury of a loved one.”

Ms. Colvin reported that regime forces had “dug a huge trench around” the Baba Amr district “and let virtually no one in or out.” Snipers posted on rooftops “shoot any civilian who comes into their sights.” And the worst may be yet to come: “About 5,000 Syrian soldiers are believed to be on the outskirts of Baba Amr, and [opposition forces] received reports . . . that they were preparing a ground assault. The residents dread the outcome.”

In her final interview with the BBC, Ms. Colvin, who covered the wars in the Balkans during the 1990s, cited the massacre of Srebrenica, where Serb troops killed thousands of civilians; she recalled the international investigations that followed and the solemn “vows to never let it happen again.”

What Ms. Colvin was trying to tell the world is that it is happening again, in Baba Amr. “No one can understand how the international community can let this happen,” she said. If the Western nations and Syria’s neighbors continue to stand by passively, Ms. Colvin’s words will come back to haunt them.