Three months into the Nigerian Civil War, in August 1967, Biafran forces had invaded the Mid-Western Region west of the Niger River, pushing as far as Benin City before being driven back by the Nigerian Second Infantry Division under Colonel Murtala Muhammed. The retreating Biafrans crossed the Niger at the bridge connecting Asaba to Onitsha, the major city on the eastern bank, and then destroyed the bridge's eastern spans to prevent pursuit. Federal troops could not cross. They were stranded on the western side, in Asaba, a predominantly Igbo town that shared ethnic ties with the Biafran southeast but lay on the federal side of the river. The people of Asaba were caught between the geography of their location and the ethnicity that Nigerian soldiers had come to treat as guilt.

Federal soldiers entered Asaba around October 5 and began killing civilians. Reports describe several hundred men killed individually and in small groups at locations across the town over the first two days. Community leaders, desperate to halt the violence, organized the procession on the morning of October 7. The idea was simple: a public, visible demonstration of allegiance to the Nigerian state. Hundreds gathered in ceremonial white and marched along the main street. At a junction, soldiers separated the crowd. Women and young children were directed one way. Men and teenage boys were gathered in an open square at Ogbe-Osowa village. Federal troops revealed machine guns. Orders to fire were given, reportedly by Major Ibrahim Taiwo, the Second Division's second-in-command. The shooting continued until most of the men and boys in the square were dead.

Most of the dead were buried in mass graves without proper ceremony. Some families were able to retrieve the bodies of their relatives and bury them at home, but for the majority, there was no funeral, no mourning ritual, no closure. Many extended families lost dozens of men and boys in a single morning. Federal troops remained in Asaba for months afterward. During the occupation, most of the town was destroyed. Women and girls were subjected to rape and forced marriages. Large numbers of residents fled, many not returning until the war ended in January 1970. Asaba became, in the words of one account, a town of women with no men. The exact death toll has never been established. In 1981, the Asaba Development Council compiled a list of 373 confirmed dead but acknowledged it was incomplete. Anthropologist S. Elizabeth Bird and historian Fraser Ottanelli estimated between 500 and 800 killed. David Scanlon of Quaker Relief Services reported 759 men and boys dead. Journalist Colin Legum put the figure at 700. Eyewitness accounts range from 500 to over 1,000.

For decades, the Asaba massacre existed primarily in the memories of survivors and their descendants, largely absent from Nigeria's official reckoning with its civil war. The Nigerian Human Rights Violations Investigations Commission, known as the Oputa Panel, heard testimony related to the events, though confusion persisted about which officers bore responsibility. I.B.M. Haruna was sometimes incorrectly identified as the officer who ordered the killings; he did not arrive in Asaba until spring 1968, when he replaced Murtala Muhammed as commanding officer of the Second Division. In October 2017, the Asaba community marked the fiftieth anniversary of the massacre with a two-day commemoration. That same year, Bird and Ottanelli published "The Asaba Massacre: Trauma, Memory, and the Nigerian Civil War" through Cambridge University Press, a work drawing on interviews with survivors and military figures as well as archival sources. The Asaba Memorial Project continues to document the massacre and its aftermath, preserving the testimonies of those who lived through it so that what happened on that morning in white cannot be forgotten.