February 25, 1866 – 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.

April 10, 1866 – The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.

May 2, 1866 – Peruvian defenders fight off Spanish fleet at the Battle of Callao.

May 16, 1866 – The U.S. Congress eliminates the half dime coin and replaces it with the five cent piece, or nickel.

May 31, 1866 – In the Fenian Invasion of Canada, John O'Neill leads 850 Fenian raiders across the Niagara River at Buffalo, New York/Fort Erie, Ontario, as part of an effort to free Ireland from the United Kingdom. Canadian militia and British regulars repulse the invaders in over the next three days, at a cost of 9 dead and 38 wounded to the Fenian’s 19 dead and about 17 wounded.

June 2, 1866 – Fenian raids: Fenians are victorious in both the Battle of Ridgeway and the Battle of Fort Erie.

July 20, 1866 – Austro-Prussian War: Battle of Lissa – The Austrian Navy , led by Admiral Wilhelm von Tegetthoff, defeats the Italian Navy near the island of Vis in the Adriatic Sea.

July 24, 1866 – Reconstruction: Tennessee becomes the first U.S. State to be readmitted to the Union following the American Civil War.

July 25, 1866 – The United States Congress passes legislation authorizing the five-star rank of General of the Army. Lieutenant General Ulysses S. Grant becomes the first to be promoted to this rank.

July 30, 1866 – New Orleans’s Democratic government orders police to raid an integrated Republican Party meeting, killing 40 people and injuring 150.

January 30, 1866 – Gelett Burgess, American literary humorist (d. 1951)

February 8, 1866 – Moses Gomberg, Russian-born American chemist (d. 1947)

February 25, 1866 – Benedetto Croce, Italian philosopher (d. 1952)

April 17, 1866 – Ernest Starling, British physiologist (d. 1927)

July 28, 1866 – Beatrix Potter, English author (d. 1943)

August 14, 1866 – Charles Jean de la Vallée-Poussin, Belgian mathematician (d. 1962)

September 7, 1866 – Tristan Bernard, French playwright and novelist (d. 1947)

September 21, 1866 – Charles Nicolle, French bacteriologist, received the Nobel Prize in Physiology or Medicine (d. 1936)

November 8, 1866 – Herbert Austin, English automobile pioneer (d. 1941)

November 13, 1866 – Abraham Flexner, American educator (d. 1959)