• Prussia decisively defeats Austria in the brief Austro-Prussian War, marking the transition from Austrian to Prussian dominance of the German Confederation.
• In Japan, a secret alliance is concluded between the tozama domains of Satsuma and Choshu, for the purpose of overthrowing the Tokugawa shogunate.
• Charles Elmer Hires invents root beer.
• Jesse James robs his first bank.

These people were born in 1866:

• Chinese revolutionary Sun Yat-Sen.
• Helen Keller's teacher Anne Sullivan.

These people died in 1866:

• American statesman Lewis Cass.
• American general and failed presidential candidate Winfield Scott.

1865 - 1866 - 1867

19th century

This is a collaboratively edited writeup. You are encouraged to assimilate and supersede it.