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1865 (MDCCCLXV) was a common year starting on Sunday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1865th year of the Common Era (CE) and Anno Domini (AD) designations, the 865th year of the 2nd millennium, the 65th year of the 19th century, and the 6th year of the 1860s decade. As of the start of 1865, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 4 – The New York Stock Exchange opens its first permanent headquarters at 10-12 Broad near Wall Street, in New York City.
- January 13 – American Civil War – Second Battle of Fort Fisher: United States forces launch a major amphibious assault against the last seaport held by the Confederates, Fort Fisher, North Carolina.
- January 15 – American Civil War: United States forces capture Fort Fisher.
- January 31
- February 3 – American Civil War – Hampton Roads Conference: Union and Confederate leaders discuss peace terms.
- February 8 & March 8 – Gregor Mendel reads his paper on Experiments on Plant Hybridization at two meetings of the Natural History Society of Brünn in Moravia, subsequently taken to be the origin of the theory of Mendelian inheritance.
- February 21 – John Deere receives a patent for ploughs.
- February 22 – Tennessee adopts a new constitution, that abolishes slavery.
- March 3 – The U.S. Congress authorizes formation of the Bureau of Refugees, Freedmen and Abandoned Lands.
- March – Hamm's Brewery opens in St. Paul, Minnesota.
- March 4 – Abraham Lincoln is sworn in for a second term as President of the United States.
- March 4 – Washington College and Jefferson College are merged, to form Washington & Jefferson College.
- March 13 – American Civil War: The Confederate States of America agrees to the use of African American troops.
- March 18 – American Civil War: The Congress of the Confederate States of America adjourns for the last time.
- March 19–21 – American Civil War – Battle of Bentonville: Union troops compel Confederate forces to retreat from Four Oaks, North Carolina.
- March 25
- The Claywater Meteorite explodes just before reaching ground level in Vernon County, Wisconsin; fragments having a combined mass of 1.5 kg (3.3 lb) are recovered.
- American Civil War: In Virginia, Confederate forces capture Fort Stedman from the Union. Lee's army suffers heavy casualties: about 2,900, including 1,000 captured in the Union counterattack. Confederate positions are weakened. After the battle, Lee's defeat is only a matter of time.
- April 1 – American Civil War – Battle of Five Forks: In Petersburg, Virginia, Confederate General Robert E. Lee begins his final offensive.
- April 2 – American Civil War: Confederate President Jefferson Davis and most of his Cabinet flee the Confederate capital of Richmond, Virginia, which is taken by Union troops the next day.
- April 6 – German chemicals producer Badische Anilin- und Sodafabrik (BASF) is founded in Mannheim.
- April 9 – American Civil War: Confederate States Army General Robert E. Lee surrenders to Union Army General Ulysses S. Grant at Appomattox Court House, effectively ending the American Civil War.
- April 14
- April 15 – President Lincoln dies early this morning from his gunshot wound. Vice President Andrew Johnson becomes the 17th President of the United States, upon Lincoln's death. Johnson is sworn in later that morning.
- April 18 – Confederate President Jefferson Davis and his entire cabinet arrive in Charlotte, North Carolina, with a contingent of 1,000 soldiers.
- April 21 – German Chemicals producer BASF moves its headquarters and factories from Mannheim, to the Hemshof District of Ludwigshafen.
- April 26
- April 27
- May 1 – The Triple Alliance of Argentina, Brazil, and Uruguay against Paraguay is formally signed; the Paraguayan War has already begun.
- May 4 – American Civil War: Lieutenant General Richard Taylor, commanding all Confederate forces in Alabama, Mississippi, and eastern Louisiana, surrenders his forces to Union General Edward Canby at Citronelle, Alabama, effectively ending all Confederate resistance east of the Mississippi River.
- May 5
- May 10 – American Civil War: Jefferson Davis is captured by the Union Army near Irwinville, Georgia.
- May 12–13 – American Civil War – Battle of Palmito Ranch: In far south Texas, more than a month after Confederate General Lee's surrender, the last land battle of the civil war with casualties, ends with a Confederate victory.
- May 17
- May 23 – Grand Review of the Armies: Union Army troops parade down Pennsylvania Avenue (Washington, D.C.) to celebrate the end of the American Civil War.
- May 25 – Mobile magazine explosion: 300 are killed in Mobile, Alabama, when an ordnance depot explodes.
- May 28 – The Mimosa sets sail with emigrants from Wales for Patagonia.
- May 29 – American Civil War: President of the United States Andrew Johnson issues a proclamation of general amnesty for most citizens of the former Confederacy.
- June–August – English polymath Francis Galton formulates eugenics.
- June 2 – American Civil War: Confederate forces west of the Mississippi River under General Edmund Kirby Smith surrender at Galveston, Texas, under terms negotiated on May 26, becoming the last to do so.
- June 10 – Richard Wagner's opera Tristan und Isolde debuts at the Munich Court Theatre.
- June 11 – Battle of the Riachuelo: The Brazilian Navy squadron defeats the Paraguayan Navy.
- July – The Christian Mission, later renamed The Salvation Army, is founded in Whitechapel, London, by William and Catherine Booth.
- July 4 – Lewis Carroll publishes his children's novel Alice's Adventures in Wonderland in England (first trade editions in December).
- July 5
- July 7 – Following Abraham Lincoln's assassination on April 14, the four conspirators condemned to death during the trial are hanged, including David Herold, George Atzerodt, Lewis Payne and Mary Surratt. Her son, John Surratt, escapes execution by fleeing to Canada, and ultimately to Egypt.
- July 14 – First ascent of the Matterhorn: The summit of the Matterhorn in the Alps is reached for the first time, by a party of 7 led by the Englishman Edward Whymper; 4 die in a fall during the descent.
- July 21 – Wild Bill Hickok – Davis Tutt shootout: In the market square of Springfield, Missouri, Wild Bill Hickok shoots Little Dave Tutt dead over a poker debt, in what is regarded as the first true western fast draw showdown.
- July 23 – The SS Great Eastern departs on a voyage to lay a transatlantic telegraph cable.
- July 27 – Welsh settlers arrive in Argentina at Chubut Valley.
- July 30 – The steamer Brother Jonathan sinks off the California coast, killing 225.
- July 31 – The first narrow gauge mainline railway in the world opens at Grandchester, Australia.
- August 16 – The Dominican Republic regains independence from Spain.
- August 25 – The Shergotty meteorite Mars meteorite falls in Sherghati, Gaya, Bihar, India.
- September 19 – Union Business College (now Peirce College) is founded in Philadelphia.
- September 26 – Champ Ferguson becomes the first person (and one of only two) to be convicted of war crimes for actions taken during the American Civil War, found guilty by a U.S. Army tribunal on 23 charges, arising from the murder of 53 people. He is hanged on October 20, two days after the conviction of Henry Wirz for war crimes.
- October 11 – Morant Bay rebellion: Paul Bogle leads hundreds of black men and women in a march in Jamaica; the rebellion is brutally suppressed by the British governor Edward John Eyre with 400 executed.
- October 25 – Florida drafts its constitution in Tallahassee.
- October 26
- The Standard Oil Company opens.
- The paddlewheel steamer SS Republic sinks off the Georgia coast, with a cargo of $400,000 in coins.
- November 6 – American Civil War: Confederate captain James Waddell surrenders the commerce raider CSS Shenandoah to the British at Liverpool.
- November 10 – Captain Henry Wirz, Confederate superintendent of Andersonville Prison (Camp Sumter) is hanged, becoming the second of two combatants, and only serving regular soldier, to be executed for war crimes committed during the American Civil War.
- November 11 – Duar War between Britain and Bhutan ends with the Treaty of Sinchula, in which Bhutan cedes control of its southern passes to Britain in return for an annual subsidy.
- November 26 – Battle of Papudo: The Spanish ship Covadonga is captured by the Chileans and the Peruvians, north of Valparaíso, Chile.
- December 11 – The United States Congress creates the United States House Committee on Appropriations and the Committee on Banking and Commerce, reducing the tasks of the House Committee on Ways and Means.
- December 17 – Leopold II becomes King of the Belgians, following the death (on December 10) of his father, King Leopold I.
- December 18 – Secretary Seward declares the Thirteenth Amendment to the United States Constitution ratified by three-quarters of the states (including those in secession) as of December 6; slavery is legally outlawed in the last two slave states of Kentucky and Delaware, and the remaining 45,000 slaves are freed.
- December 21 – The Kappa Alpha Order is founded at Washington College, Lexington, Virginia.
- December 24 – Jonathan Shank and Barry Ownby form the Ku Klux Klan in the American South, to resist Reconstruction and intimidate carpetbaggers and scalawags, as well as to repress the freedpeople.