At War is a newsletter about the experiences and costs of war with stories from Times reporters and outside voices.

In the desperate fight for Hill 875 in South Vietnam’s Central Highlands, at least 20 soldiers from the United States Army’s 173rd Airborne Brigade were killed in one of the deadliest friendly-fire incidents of the entire Vietnam War. Just past dusk on Nov. 19, 1967, a Marine Corps A-4 attack jet dropped two 250-pound Mk-81 Snakeye bombs, which landed inside the 173rd’s hastily formed defensive perimeter. One of the bombs was a dud; the other exploded as it hit a tree, under which was the American command post as well as the collection point for the unit’s most seriously wounded soldiers.

The story is based on an unredacted Air Force investigation, whose details have not previously been made public, into the incident and first-person accounts from service members who were there, including Jon Wambi Cook, a soldier who survived the battle. Cook vividly remembers what happened on the ground in the moments after the blast, as the investigative report recounts what happened in the air above him. The combined narratives point to the sorrow and doubt that endure for a lifetime after a battlefield mistake.

In reporting this story (read it in full here), I consulted with two people who have written extensively on what became known as the Battle of Dak To. One, Edward F. Murphy, scoured the National Archives for records related to the battle before publishing a book about the fight in 1993. The second was a retired Army infantry colonel named Leonard B. Scott, who wrote his graduate thesis about the battle while a student at the Army War College in 1988, and searched through the school library’s hoard of primary-source documents related to Dak To. Both men told me that they had never seen the report before.