He rented houses around the main square (Plaza Mayor) in secret, and from there they excavated tunnels to mine the Fort.

The British spy network only heard about this place a few days before the start of the reaction, and was too late to attack Perdriel, only accelerating the fight for reconquest.

When Santiago de Liniers arrived back from Montevideo and started the Reconquest, on 12 August, he was joined by Álzaga's secret army, and the British were rapidly defeated.

The British fleet had not left the Río de la Plata, and awaited reinforcements which arrived under the command of General Whitelocke.

He participated in the organization of volunteer city militias, and army of more than 6,000 men, and paid for a regiment of Asturians and Vizcayans out of his own funds.

Álzaga took his regiments to the streets (the "Gallegos", "Miñones de Cataluña" and "Vizcaínos", all Spaniards), organized a protest against the viceroy and tried to force Liniers to resign.

Álzaga took part in the subsequent revolution against Cisneros the following year, and even though he was not present in the open cabildo of May 22, 1810, it is known he participated in the negotiations to leading to the formation of the Primera Junta, as he placed three members of his party: Mariano Moreno, Juan Larrea and Domingo Matheu.

During the ensuing investigation, Secretary Rivadavia, based on dubious proof and confessions extended the accusations against Álzaga and his co-conspirators.

The executions started on July 4, two days after his arrest which raises the suspicion that the trial outcome was previously decided.