FIVE years ago, at roughly 4:30 p.m. on Aug. 19, 2003, in Baghdad, a suicide bomber in a flatbed truck pulled up outside the lightly fortified office of the United Nations’s leading diplomat, Sergio Vieira de Mello, and detonated a cone-shaped bomb the size of a large man. The bomb was laced with hand grenades and mortars, and it carried more than a thousand pounds of explosives. Its force was so fierce that it shook the American-controlled Green Zone, three miles away.

While many United Nations officials were killed instantly, Mr. Vieira de Mello was not. For more than three hours, he lay trapped beneath the collapsed roof and floors of the three-story building, as he asked about the fates of his colleagues and complained about the pain in his legs. Although the Bush administration had not equipped American forces to respond to large-scale terrorist attacks on civilian targets, several soldiers heroically risked their lives to save him, submerging themselves in the sweltering, crumbling wreckage.

But without proper equipment, they were forced to rely on their hands and helmets, along with a makeshift pulley system constructed out of a woman’s straw handbag and an office curtain rope. Mr. Vieira de Mello died, as did 21 other people from 11 countries. Among the dead were experts in conflict resolution, humanitarian assistance and development work. More than 150 people were severely injured. Survivors of the attack and devastated colleagues branded the day the United Nations’s 9/11.

Just as we Americans tried to make sense of our tragedy, United Nations officials, nongovernmental workers and world leaders grappled with applying the lessons of August 19. But five years later  and less than a week after Taliban forces in Afghanistan killed three female educators and a driver with the International Rescue Committee  the individuals who carry out vital humanitarian and development work for the United Nations and nongovernmental organizations have never been more at risk.