Events in the year 1866 in Argentina.
Incumbents President: Bartolomé Mitre Vice President: Marcos Paz
Governors Buenos Aires Province: Mariano Saavedra (until 1 May), Adolfo Alsina (starting 1 May) Mendoza Province: until 1 November: Carlos González 1 November-11 November: Melitón Arroyo starting 11 November: Carlos Juan Rodríguez Santa Fe Province: Nicasio Oroño
Vice Governors Buenos Aires Province: vacant
Events January 31 – Paraguayan War: The Battle of Pehuajó takes place when about 1500 Paraguayan troops, commanded by General Francisco Isidoro Resquín and Lieutenant Celestino Prieto, engage in a surprise attack Argentinian and Uruguayan battalions with around 2000 men, led by General Emilio Conesa; the Paraguayans are successful and the joint Argentine-Uruguayan force suffers heavy casualties.O'Leary, Juan; El Libro de los Héroes; Editorial Servilibro; Asunción, Paraguay (2007) May 24 – Paraguayan War: First Battle of Tuyutí is an allied victory.Williams, John Hoyt (2000), "A Swamp of Blood: The Battle of Tuyuti", Military History 17 (1): 58–64. July 18 – Battle of Boquerón (1866) September 1–3 – Battle of Curuzú September 22 – Battle of Curupayty
Deaths February 15 – Juan Gregorio de las Heras, military leader (born 1780)
References