During the political unrest of the 1850s, Zaragoza joined the army supporting the cause of the Liberal Party, in opposition to dictator Antonio López de Santa Anna. Zaragoza rose to command an army of volunteers that in 1855 defeated Santa Anna and led to the re-establishment of a constitutional democratic government in Mexico.
Zaragoza served as Secretary of War from April through October 1861, in the cabinet of Benito Juárez. He resigned in order to lead the Army of the East (Ejército de Oriente) against the Europeans who, using the Mexican external debt as a pretext under the Treaty of London concluded earlier that year, had invaded Mexico.
