Here are some of the most important events of 1866, presented in a Wikipedia article style:

1866

1866 was a year of significant political, social, and technological developments, particularly in the wake of the American Civil War.

Political Events

• April 9: The United States Congress passes the Civil Rights Act of 1866. This landmark legislation, primarily aimed at protecting the rights of African Americans following the Civil War, declared that all persons born in the United States (except Native Americans) were citizens, regardless of race or previous condition of servitude. President Andrew Johnson vetoed the bill, but Congress overrode his veto, marking a crucial victory for Radical Reconstruction.
• May 16: The Tenure of Office Act is passed by the United States Congress. This act restricted the power of the President to remove certain office-holders without Senate approval. It was a direct response to President Johnson's actions and aimed to protect Edwin Stanton, the Secretary of War. This Act will play a key role in Johnson's impeachment later on.
• June 13: The United States Congress approves the Fourteenth Amendment to the United States Constitution. This amendment, a cornerstone of Reconstruction, guarantees equal protection under the law and due process to all citizens, and addresses issues of citizenship and representation. It was ratified in 1868.
• June 20: The Third Italian War of Independence begins as Prussia declares war on Austria, as part of the wider Austro-Prussian War. Italy aligns itself with Prussia, hoping to acquire Venetia from Austrian control.
• July 3: The Battle of Königgrätz (also known as the Battle of Sadowa) results in a decisive Prussian victory over Austria in the Austro-Prussian War. This victory significantly shifted the balance of power in Central Europe and led to the formation of the North German Confederation.
• August 23: The Treaty of Prague is signed, officially ending the Austro-Prussian War. Austria ceded Venetia to Italy (though the Italians had not won any major battles). The German Confederation was dissolved, and Prussia became the dominant power among the German states.
• November Midterm Elections (USA): Radical Republicans gain a significant majority in both houses of the United States Congress. This gives them the power to enact their Reconstruction agenda over President Johnson's objections.

Scientific and Technological Events

• January 20: First official building society is founded in Britain.
• July 27: Successful completion of the transatlantic telegraph cable laying between Foilhommerum, Valentia Island, Ireland, and Heart's Content, Newfoundland. This facilitated near-instant communication between Europe and North America. This event follows a failed attempt the previous year.
• Werner von Siemens invents the dynamo-electric machine, a crucial development in the field of electrical engineering.
• John Langdon Down publishes a description of Down Syndrome, identifying it as a distinct clinical entity.

Cultural Events

• Fyodor Dostoevsky publishes Crime and Punishment, a major work of Russian literature exploring moral and psychological themes.
• Publication of Little Women by Louisa May Alcott. The first volume of this beloved novel for young adults was published, and it met with immediate success.

Other Notable Events

• October 12: The United Kingdom grants limited self-government to Jamaica.