These events followed a 1955 military coup, known as Revolución Libertadora ('the Liberating Revolution'), which deposed Argentine president Juan Domingo Perón and eventually brought the right-wing dictatorship to power, led first by Lieutenant General Eduardo Lonardi – who was considered too 'lenient' (with Peronism) by the leaders of the coup and quickly deposed – and later by the hard-line General Pedro Eugenio Aramburu. Walsh claims that the men were arrested before the establishment in that very same night of the martial law and that they also were never properly charged, therefore they were unlawfully shot.

The book details the José León Suárez massacre, which involved the Junlawful capture and shooting by the Buenos Aires Province Police of a group of civilians suspected of being involved with a Peronist uprising that same night, including the rebel leader General Juan José Valle. The most recent editions of the book in Spanish and English also include additions (listed as 'Appendices') to the text written by Walsh for the various editions of the book that came out after its first publication in 1957. The book is divided into three sections: in the first, Walsh provides portraits of the victims of the shooting in the second, he reconstructs the events of the night in question in the third, he shares testimonies from the head of the Buenos Aires Province Police, among others, that unequivocally betray the complicity of the de facto State and make a case for the unlawful and disgraceful execution of the men in question.

It was published in 1957, nine years before the publication of Truman Capote's In Cold Blood, a book often credited as the first major nonfiction novel of investigative journalism.

It is considered by some to be the first of its genre. Operación Masacre (English: 'Operation Massacre') is a nonfiction novel of investigative journalism, written by noted Argentine journalist and author Rodolfo Walsh.

Operación Masacre by Rodolfo Walsh