12th January » The Royal Aeronautical Society is formed in London.
16th February » Spencer Compton Cavendish, 8th Duke of Devonshire e.g Spencer Compton Cavendish, Marquess of Hartington becomes British Secretary of State for War.
25th February » Miners in Calaveras County, California, discover what is now called the Calaveras Skull - human remains that supposedly indicated that man, mastodons, and elephants had co-existed.
31st March » The Spanish Navy bombs the harbor of Valparaíso, Chile.
4th April » Alexander II of Russia narrowly escapes an assassination attempt by Dmitry Karakozov in the city of Saint Petersburg.
6th April » The Grand Army of the Republic, an American patriotic organization composed of Union veterans of the American Civil War, is founded. It lasts until 1956.
8th April » Italy and Prussia Austro-Prussian War named ally against the Austrian Empire.
10th April » The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.
1st May » The Memphis Riots of 1866 named Memphis Race Riots begin. In three days time, 46 blacks and two whites were killed. Reports of the atrocities influenced passage of the Fourteenth Amendment to the United States Constitution.
2nd May » Peruvian defenders fight off the Spanish fleet at the Battle of Callao.
5th May » Memorial Day first celebrated in United States at Waterloo, New York.
10th May » Romania National Holiday 1866-1947, The Modern Monarchy Instauration of the Kingdom of Romania, Carol I of Romania
16th May » The United States Congress e.g U.S. Congress eliminates the half dime coin and replaces it with the five cent piece, or nickel (U.S. coin) named nickel.
2nd June » Fenian raids: the Fenian Brotherhood or Fenians are victorious over Canadian forces in both the Battle of Ridgeway and the Battle of Fort Erie (1866) known as Battle of Fort Erie.
3rd June » The Fenians are driven out of Fort Erie, Ontario, into the United States.
7th June » 1,800 Fenian raiders are repelled back to the United States after they looted and plundered around Saint-Armand, Quebec called Saint-Armand and Frelighsburg, Quebec.
24th June » Battle of Custoza (1866) Battle of Custoza: an Austrian Empire Austrian army defeats the Italy Italian army during the Austro-Prussian War.
3rd July » Austro-Prussian War is decided at the Battle of Königgratz, resulting in Prussia taking over as the prominent German nation from Austria.
20th July » Austro-Prussian War: Battle of Lissa (1866) Battle of Lissa The Austro-Hungarian Navy Austrian Navy , led by Admiral Wilhelm von Tegetthoff, defeats the Regia Marina Italian Navy near the island of Vis (island) Vis in the Adriatic Sea.
24th July » Reconstruction Era of the United States named Reconstruction: Tennessee becomes the first U.S. state to be readmitted to the Union following the American Civil War.
25th July » The United States Congress passes legislation authorizing the Military rank rank of General of the Army (United States) General of the Army. Lieutenant general (United States) Lieutenant General Ulysses S. Grant becomes the first to be promoted to this ra
28th July » At the age of 18, Vinnie Ream becomes the first and youngest female artist to receive a commission from the United States government for a statue (of Abraham Lincoln).
17th August » The Grand Duchy of Baden announces her withdrawal from the German Confederation and signs a History of Baden-Württemberg named treaty of peace and alliance with Prussia.
20th August » President of the United States or President Andrew Johnson formally declares the American Civil War over.
23rd August » Austro-Prussian War ends with the Treaty of Prague (1866) named Treaty of Prague.
22nd September » Battle of Curupayty in the Paraguayan War.
19th October » Venice - Annexion of Veneto and Mantua to Italy - At Hotel Europa, Austria hands over Veneto to France, which hands it immediately over to Italy.
22nd October » A plebiscite ratifies the annexion of Veneto and Mantua to Italy, occurred three days before, on October 19.