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1870 (MDCCCLXX) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Thursday of the Julian calendar, the 1870th year of the Common Era (CE) and Anno Domini (AD) designations, the 870th year of the 2nd millennium, the 70th year of the 19th century, and the 1st year of the 1870s decade. As of the start of 1870, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 1
- January 3 – Construction of the Brooklyn Bridge begins.
- January 6 – The Musikverein, Vienna, is inaugurated in Austria-Hungary.
- January 10 – John D. Rockefeller incorporates Standard Oil.
- January 15 – A political cartoon for the first time symbolizes the United States Democratic Party with a donkey (A Live Jackass Kicking a Dead Lion by Thomas Nast for Harper's Weekly).
- January 20 – SS City of Boston vanishes at sea, with all 177 aboard.
- January 23 – Marias Massacre: U.S soldiers attack a peaceful camp of Piegan Blackfeet Indians, led by chief Heavy Runner.
- January 26 – Reconstruction Era (United States): Virginia rejoins the Union.
- February – Denis Vrain-Lucas is sentenced to 2 years in prison, for multiple forgery, in Paris.
- February 1 – Goodna State School in Goodna, Queensland, Australia is founded.
- February 2 – It is revealed that the famed Cardiff Giant in the U.S. is just carved gypsum, and not the petrified remains of a human.
- February 3 – The 15th Amendment to the United States Constitution, guaranteeing African-Americans the right to vote, is passed.
- February 9 – The U.S. Army Weather Bureau (within the Army Signal Corps) is created.
- February 10
- February 12 – Women's suffrage]: Women gain the right to vote in Utah Territory.
- February 23 – Military control of Mississippi ends, and it is readmitted to the Union.
- February 25 – Hiram Rhodes Revels, a Republican from Mississippi, is sworn into the United States Senate, becoming the first African American ever to sit in the U.S. Congress.
- February 26 – In New York City, the first pneumatic subway is opened, Beach Pneumatic Transit.
- February 26 – The German Commerzbank is founded in Hamburg.
- February 27 – The circle of the sun flag of Japan is adopted as the national flag for Japanese merchant ships, by proclamation of the Daijō-kan.
- February 28 – The Bulgarian Exarchate is established, by decree of Sultan Abdülaziz of the Ottoman Empire.
- March – The Mitsubishi Company is established in Japan as a shipping firm, by Iwasaki Yatarō with Thomas Blake Glover.
- March 1 – Battle of Cerro Corá, Paraguay: Marshal Francisco Solano López's last troops are cornered by those of the Triple Alliance. López refuses to surrender and is killed, ending the Paraguayan War.
- March 4 – Red River Rebellion: Thomas Scott is executed by Louis Riel's provisional government, in modern-day Manitoba, Canada.
- March 5 – The first ever international Association football match, England v Scotland, takes place under the auspices of the Football Association at The Oval, London.
- March 10 – The Deutsche Bank is founded in Berlin.
- March 19 – The Ohio Legislature passes the Cannon Act, thereby establishing the Ohio Agriculture and Mechanical College, later Ohio State University.
- March 24 – Syracuse University is established and officially opens.
- March 30
- March 31 – Thomas Mundy Peterson is the first African-American to vote in an election.
- July 13 – The Ems Dispatch serves as a reason for a war between Prussia and France.
- July 15
- July 18 – Pastor aeternus: Pope Pius IX declares papal infallibility, in matters of faith and morals.
- July 19 – Franco-Prussian War: France declares war on Prussia.
- July 30 – The Klipdrift Republic is proclaimed in South Africa by diamond miners, with Stafford Parker as President.
- August 2 – The Tower Subway beneath the River Thames in London, the world's first underground passenger "tube" railway, officially opens. Although this lasts as a railway operation only until November, it demonstrates the technologically successful first use of the cylindrical wrought iron tunnelling shield, devised by Peter W. Barlow and James Henry Greathead, and of a permanent tunnel lining of cast iron segments.
- August 8 – The Republic of Ploiești, an uprising against Domnitor Carol of Romania, fails.
- August 24 – The Red River Rebellion, ends with the arrival of the Wolseley Expion, and the fleeing of Louis Riel.
- September 2 – Franco-Prussian War – Battle of Sedan: Prussian forces defeat the French armies, and take Emperor Napoleon III and 100,000 of his soldiers prisoner at Sedan.
- September 4 – Emperor Napoleon III of France is deposed, and the Third Republic is declared. Empress Eugénie flees to England with her children.
- September 6 – Louisa Ann Swain of Laramie, Wyoming, becomes the first woman in the United States to cast a vote legally since 1807.
- September 18 – Old Faithful Geyser is observed and named by Henry D. Washburn, during the Washburn–Langford–Doane Expion to Yellowstone.
- September 19 – Franco-Prussian War: The Siege of Paris begins.
- September 20 – With Bersaglieri soldiers entering Rome at Porta Pia, the unification of Italy is completed, ending the last remnant of the Papal States and the Papal temporal power.
- October 2 – A plebiscite held in Rome supports, by 133,681 votes to 1,507, the annexation of the city by Italy.
- October 6 – Rome becomes the capital of unified Italy.
- October 8 – Léon Gambetta escapes the besieged Paris in a hot-air balloon.
- October 20 – The First Vatican Council adjourns.
- October 27 – Franco-Prussian War – Siege of Metz: Marshal François Achille Bazaine, commanding the French left wing, is forced by starvation to surrender the fortifications of Metz.
- November 1 – In the United States, the newly created Weather Bureau (later renamed the National Weather Service) makes its first official meteorological forecast: "High winds at Chicago and Milwaukee... and along the Lakes".
- November 12 – German company Dresdner Bank is founded.
- November 16 – The Spanish Cortes Generales proclaims Amadeo de Saboya, as King Amadeus I of Spain.
- December 12 - Joseph H. Rainey of South Carolina becomes the second black U.S. congressman, the first being Hiram Revels.
- December 30 – Juan Prim, prime minister of Spain, is assassinated.
- December 31
- January 1 – Hermann Theodor Simon, German physicist (d. 1918)
- January 2 – Ernst Barlach, German sculptor, graphic artist and poet (d. 1938)
- January 6 – Gustav Bauer, Chancellor of Germany (d. 1944)
- January 8
- January 11 – Alexander Stirling Calder, American sculptor (d. 1945)
- January 20 – Ajahn Mun Bhuridatta, Thai Buddhist monk (d. 1949)
- January 22 – John B. Sheridan, Irish American sports journalist (d. 1930)
- January 23 – William G. Morgan, American inventor of volleyball (d. 1942)
- February 7 – Alfred Adler, Austrian psychologist (d. 1937)
- February 12
- February 20 – Jay Johnson Morrow, American military engineer, politician, 3rd Governor of the Panama Canal Zone (d. 1937)
- February 25 – Jelica Belović-Bernardzikowska, Croatian writer (d. 1946)
- March 4 – Thomas Sturge Moore, English poet, author and artist (d. 1944)
- March 5 – Frank Norris, American writer (d. 1902)
- March 13 – Seale Harris, American physician (d. 1957)
- March 17 – Horace Donisthorpe, English entomologist (d. 1951)
- March 20 – Paul von Lettow-Vorbeck, German general (d. 1964)
- March 31 – James M. Cox, Democratic candidate for President of the United States in the election of 1920 (d. 1957)
- April 1 – Hamaguchi Osachi, 27th Prime Minister of Japan (d. 1931)
- April 3 – Agda Östlund, Swedish politician (d. 1942)
- April 4
- April 7 – Anna Lindhagen, Swedish politician (d. 1941)
- April 17 – Ray Stannard Baker, American journalist, author (d. 1946)
- April 22 – Vladimir Lenin, Russian revolutionary, first Premier of the Soviet Union (d. 1924)
- April 21 – Edwin S. Porter, American film director (d. 1941)
- April 30 – Franz Lehár, Austrian composer (d. 1948)
- May 4 – Antonius van den Broek, Dutch physicist (d. 1926)
- May 9 – Harry Vardon, English golf professional (d. 1937)
- May 19 – Albert Fish, American serial killer (d. 1936)
- May 24
- May 27 – Anna Stecksén, Swedish scientist, physician and pathologist (d. 1904)
- June 13 – Jules Bordet, Belgian immunologist, microbiologist, recipient of the Nobel Prize in Physiology or Medicine (d. 1961)
- June 20 – Georges Dufrénoy, French post-impressionnist painter (d. 1943)
- July 3 – Richard Bedford Bennett, eleventh Prime Minister of Canada (d. 1947)
- July 9 – Mathew Beard, American supercentenarian (d. 1985)
- July 12 – Louis II, Prince of Monaco (d. 1949)
- July 16 – Lambert McKenna, Irish scholar (d. 1956)
- July 25 – Maxfield Parrish, American illustrator (d. 1966)
- July 27 – Hilaire Belloc, French/English man of letters (d. 1953)
- July 29 – George Dixon, Canadian boxer (d. 1909)
- August 3 – Carrie Ingalls, younger sister of author Laura Ingalls Wilder (d. 1946)
- August 4 – Harry Lauder, Scottish entertainer (d. 1950)
- August 10 – Hans Zenker, German admiral (d. 1932)
- August 11 – Tom Richardson, English cricketer (d. 1912)
- August 12 – Hubert Gough, British general (d. 1963)
- August 14 – Nelson McDowell, American actor (d. 1947)
- August 20 – Edward Stanley Kellogg, 16th Governor of American Samoa (d. 1948)
- August 22 – Bertram Fletcher Robinson, English journalist, or and author (d. 1907)
- August 31 – Maria Montessori, Italian educator (d. 1952)
- September 1 – Leopold Wharton, American film director (d. 1927)
- September 2 – James Bert Garner, American chemical engineer, inventor (d. 1960)
- September 24 – Georges Claude, French engineer, inventor (d. 1960)
- September 25 – James A. Hawken, American schoolteacher (d. 1964)
- September 26 – King Christian X of Denmark (d. 1947)
- September 30
- October 2 – Horace Hood, British admiral (d. 1916)
- October 4 – Karl Renner, 1st Chancellor of Austria (d. 1950)
- October 10 – Ivan Alekseyevich Bunin, Russian writer, Nobel Prize laureate (d. 1953)
- October 18 – D. T. Suzuki, Japanese philosopher (d. 1966)
- October 22 – Johan Ludwig Mowinckel, Norwegian businessman, Prime Minister of Norway (d. 1943)
- October 30 – Lawrence Grant, English actor (d. 1952)
- November 2 – Joseph "Sport" Sullivan, American gambler (d. 1949)
- November 17 – Jean Prévost, Canadian politician (d. 1915)
- November 21 – Sigfrid Edström, Swedish sports official (d. 1964)
- November 27 – Juho Kusti Paasikivi, Prime Minister and President of Finland (d. 1956)
- November 28 – Gustavus M. Blech, German-American physician, surgeon (d. 1949)
- November 29 – Trixie Friganza, American actress (d. 1955)
- December 5 – Vítězslav Novák, Czech composer (d. 1949)
- December 10 – Mary Bonaparte, pretender to the French imperial throne (d. 1947)
- December 12 – Walter Benona Sharp, American oil pioneer (d. 1912)
- December 14
- December 18 – Saki, English writer (k. 1916)
- January 25 – Victor de Broglie, Prime Minister of France (b. 1785)
- January 29 – Leopold II, Grand Duke of Tuscany (b. 1797)
- February 7 – Sylvain Salnave, Haitian general, 9th President of Haiti (b. 1827)
- February 11 – Carlos Soublette, 2-time President of Venezuela (b. 1789)
- February 19 – Nathaniel de Rothschild, French wine grower (b. 1812)
- March 1 – Francisco Solano López, 2nd President of Paraguay (killed in action) (b. 1827)
- March 4 – Thomas Scott, Canadian Orangeman, surveyor of the Red River Rebellion (shot by Louis Riel and the Métis) (b. c. 1842)
- March 11 – Moshoeshoe I of Lesotho (b. 1786?)
- March 28 – George Henry Thomas, American general (b. 1816)
- April 16 – Domnița Rallou Caragea, Greek princess, independence activist (b. 1799)
- May 6 – Sir James Young Simpson, Scottish physician, researcher (b. 1811)
- June 6 – Ferdinand von Wrangel, Baltic-German explorer (b. 1796/1797)
- June 7 – Friedrich Hohe, German lithographer, painter (b. 1802)
- June 9 – Charles Dickens, British novelist (b. 1812)
- June 20 – Jules de Goncourt, French writer, publisher (b. 1830)
- June 24 – Adam Lindsay Gordon, Australian poet (b. 1833)
- July 10 – Pelaghia Roșu, Romanian heroine (b. 1800)
- August 14 – David Farragut, American admiral (b. 1801)
- August 17 – Pedro Figueredo, Cuban poet, musician and freedom fighter (b. 1818)
- September 4 – Juan Javier Espinosa, 9th President of Ecuador (b. 1815)
- September 12 – Fitz Hugh Ludlow, American author, explorer (b. 1836)
- September 23 – Prosper Mérimée, French writer (b. 1803)
- September 27 – William F. Packer, American politician (b. 1807)
- October 12
- November 3 – Diego Noboa, 4th President of Ecuador (b. 1789)
- November 23 – Giuseppina Bozzacchi, Milanese-born ballerina (b. 1853) (result of deprivation during Siege of Paris)
- November 24 – Comte de Lautréamont, French poet, writer (b. 1846)
- November 26 – Franz Graf von Wimpffen, Austrian general and admiral (b. 1797)
- November 28 – Frédéric Bazille, French painter (b. 1841)
- December 5 – Alexandre Dumas, père, French author (b. 1802)
- December 9 – Patrick MacDowell, Northern Irish sculptor (b. 1799)
- December 27 – Juan Prim, Spanish general and prime minister (assassinated) (b. 1814)