| 1845–1958 | Germany [earth sciences] | German naturalist and explorer Alexander von Humboldt lays the basis of modern geography with the publication of Kosmos/Cosmos, in which he arranges geographic knowledge in a systematic fashion. |
| 1861–1870 | USA, UK, Ireland [statistics and demography] | Emigration to the USA from Britain totals 606,896; from Ireland it is 435,779. |
| 1866 | USA [technology] | US scientist Mahlon Loomis transmits the first telegraph message over radio waves. It is sent between two mountains in West Virginia using kites to support the aerials. |
| 1866 | USA [trade] | Three-fourths of US imports are manufactured goods, while three-fourths of US exports are crude materials or crude foodstuffs. |
| 1866 | Russia [fiction] | The Russian writer Fyodor Mikhaylovich Dostoyevsky publishes his novel Prestupleniye i nakazaniye/Crime and Punishment, the first of his major novels. |
| 1866 | Switzerland [food and drink] | Swiss company Nestlé launches its first product, formula milk for babies. |
| 1866 | Germany [biology] | The German embryologist Ernst Haeckel proposes a third category of living beings intermediate between plants and animals. Called Protista, it consists mostly of microscopic organisms such as protozoans, algae, and fungi. |
| 1866 | Austria [biology] | Austrian monk and botanist Gregor Mendel publishes ‘Versuche uber Pflanzenhybriden/Experiments on Plant Hybridization’ in the little known Proceedings of the Brunn Natural History Society in which he describes the inheritance of different characteristics in pea plants and proposes general methods for predicting patterns of inheritance, from one generation to the next. The work establishes the fundamental laws of heredity and is the basis of modern genetics. Mendel sends the paper to eminent biologists but is ignored. |
| 1866–1867 | UK [boxing] | A new set of boxing rules is drafted in Britain under the auspices of John Sholto Douglas, Marquess of Queensberry. With their insistence on the wearing of padded gloves, three-minute rounds, and a count of ten for knockouts, they herald the beginning of modern boxing. |
| 14 January 1866 | Peru, Spain [wars] | Peru declares war on Spain in resentment over clauses in the treaty of 27 January 1865 by which Peru's independence was recognized. |
| 8 April 1866 | Italy, Prussia [political events] | An offensive and defensive alliance is signed between Prussia and Italy. |
| 7 June 1866 | Prussia, Austrian Empire [Seven Weeks War (1866)] | Prussian troops march into the Austrian-ruled duchy of Holstein. The Prussian chancellor, Otto von Bismarck, wants to provoke war with Austria and end its influence in Germany. |
| 12 June 1866 | Austrian Empire, France [treaties] | A secret treaty is agreed between Austria and France, by which the French emperor, Napoleon III, promises French neutrality in Austria's coming war with Prussia provided that Austria cedes Venice, which France will in turn hand over to Italy. |
| 15 June - 16 June 1866 | Prussia, Saxony, Hanover, Germany [Seven Weeks War (1866)] | Prussia invades the German states of Saxony, Hanover, and Hesse, which, as Austria's allies, have opposed Prussia's entry into the Austrian-ruled duchy of Holstein. |
| 3 July 1866 | Prussia, Austrian Empire [Seven Weeks War (1866)] | The Prussians defeat the Austrians at Sadowa (Königgrätz), Bohemia, in the decisive battle of the Seven Weeks War. |
| 4 July 1866 | France, Venice, Austrian Empire [political events] | Emperor Napoleon III of France announces the cession of the Italian state of Venice by Austria following the Austrian defeat in the Seven Weeks' War with Prussia, as agreed in the treaty between Prussia and Austria of 12 June. |
| August 1866 | Japan [political events] | The Japanese shogun (military ruler) Iemochi dies and is succeeded by Yoshinobu. |
| 23 August 1866 | Prussia, Austrian Empire, Hanover, Germany [treaties] | The Peace of Prague between Prussia and Austria confirms the preliminary Peace of Nikolsburg (26 July) whereby Austria is to be excluded from Germany, while the German states of Hanover, Hesse, Nassau, and Frankfurt are to be incorporated with Prussia, the south German states are to be independent, but the states north of the River Main are to form a confederation under Prussia, which also obtains Austrian Silesia and territory from Saxony and from the south German states. |
| 2 September 1866 | Crete, Ottoman Empire, Greece [political events] | After a long period of unrest under Ottoman authority, the island of Crete revolts and decrees union with Greece. |
| 3 September 1866 | Prussia [administration] | Having defeated Austria and established Prussia as the leading power in Germany, the Prussian chancellor, Otto von Bismarck, obtains an indemnity from a grateful Prussian diet (state assembly) for having ruled without parliamentary approval of government budgets, ending the constitutional conflict that began in 1862. |
| 21 September 1866 | England [births and deaths] | H(erbert) G(eorge) Wells, English novelist, sociologist, and historian, who writes The Time Machine, The War of the Worlds, and The Invisible Man, born in Bromley, Kent, England (–1946). |
| 3 October 1866 | Austrian Empire, Italy [treaties] | The war between Austria and Italy, arising from Italy's support of Prussia against Austria in the Seven Weeks War, is formally ended by the Treaty of Vienna, with Italy receiving Venice from Austria. |
| 12 October 1866 | Scotland [births and deaths] | Ramsay MacDonald, British politician, first Labour Party prime minister of Britain 1924, prime minister again in 1929, and in a coalition government 1931–35, born in Lossiemouth, Moray, Scotland (–1937). |
| 24 October 1866 | Germany, Austrian Empire [political events] | The German Confederation is formally ended following Prussia's defeat of Austria for control of Germany. |
