Incumbents

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

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.^[1]
May 24 – Paraguayan War: First Battle of Tuyutí is an allied victory.^[2]
July 18 – Battle of Boquerón (1866)
September 1–3 – Battle of Curuzú
September 22 – Battle of Curupayty

^ O'Leary, Juan; El Libro de los Héroes; Editorial Servilibro; Asunción, Paraguay (2007)
^ Williams, John Hoyt (2000), "A Swamp of Blood: The Battle of Tuyuti", Military History 17 (1): 58–64.

v t e Years in Argentina (1800–present)
18th century	1800
19th century	1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900
20th century	1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
21st century	2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024

v t e 1866 in South America
Sovereign states	Argentina Bolivia Brazil Chile Colombia Ecuador Guyana Paraguay Peru Suriname Uruguay Venezuela
Dependencies and other territories	Falkland Islands French Guiana South Georgia and the South Sandwich Islands