https://en.m.wikipedia.org/wiki/1866_in_the_United_States
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March 13 The United States Congress overwhelmingly passes the Civil Rights Act of 1866, the first federal legislation to protect the rights of African-Americans; U.S. President Andrew Johnson vetoes the bill on March 27, and Congress overrides the veto on April 9.
April 10 The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.
May 16 The U.S. Congress approves the minting of a nickel 5-cent coin (nickel), eliminating its predecessor, the half dime.
June Red Cloud's War opens.
July 4 The Great Fire of Portland, Maine kills two and leaves 10,000 homeless in the worst fire in an American city at this time.
July 23 The Judicial Circuits Act reduces the number of United States circuit courts to nine and the number of Supreme Court justices to seven.
July 24 Reconstruction: Tennessee becomes the first U.S. state to be readmitted to the Union following the American Civil War.
July 25 The U.S. Congress passes legislation authorizing the rank of General of the Army (modern-day "5-star general"

; Lieutenant General Ulysses S. Grant becomes the first to have this rank.
July 28 The Metric Act of 1866 becomes law and legalizes the use of the metric system for weights and measures in the United States.
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