Around mid-1969, Brigadier General Roberto Viaux, commander of the First Army Division stationed in Antofagasta, asked "the opinion of the young officers" (colonels, lieutenant colonels, majors, and captains) and based on their answers, wrote a letter of petition to the Army Commander-in-chief, General Sergio Castillo, to be presented to President Eduardo Frei Montalva.

General Viaux's letter said that "national security and internal peace in our country" depended on the armed forces, and "in our judgement" the Frei administration, like its predecessors, had not concerned itself with keeping a modern army in working order, "well equipped and taking real part in important decisions".

The letter demanded that a new government policy toward the armed forces be put into effect, that senior officers' salaries be adjusted to conform to "their high social status and national responsibility" (in the letter, Viaux complained that "a general of the Republic earns less than a competent worker at the Chuquicamata copper mine", which was true), that adequate materiel be purchased, and that the higher echelons of the military hierarchy be given a prominent participatory role in the economic, political, and social development of Chile.

At noon on October 21, it was clear to the reporters covering the incident that General Viaux was counting on the covert support and sympathy of the majority of Santiago's officer corps, and that, technically, if Viaux suggested it, there was nothing to prevent the overthrow of Frei and his replacement by a military government without a shot being fired.

President Frei reacted by ending the legislative session of the congress, and declaring a state of emergency.