February - John Wisden publishes The Cricketer's Almanack for the year 1864 in England; it will go on to become the major annual cricket reference publication.
February 1 - Danish-Prussian War : 57,000 Austrian and Prussian troops cross the Eider River into Denmark.
February 17 - American Civil War: The tiny Confederate hand-propelled submarineH. L. Hunley sinks the, using a spar torpedo in Charleston Harbor, becoming the first submarine to sink an enemy ship, although the submarine and her crew of eight are also lost.
April 18 - Danish-Prussian War - Battle of Dybbøl: The Prussian army, fielding 10,000 men, defeats the Danish defending army of 9,200 at Dybbøl Mill, after an artillery bombardment from April 7-18.
* American Civil War: The Army of the Potomac, under General Ulysses S. Grant, breaks off from the Battle of the Wilderness and moves southwards.
* The clipper ship City of Adelaide is launched in Sunderland, England. By the 21st century, she will be the world's oldest surviving clipper of only two.
May 8-21 - American Civil War - Battle of Spotsylvania Court House: Some 4,000 troops die in an inconclusive engagement.
* Danish-Prussian War - Battle of Heligoland: The Danish navy gains a tactical victory over those of Austria and Prussia, near the island of Heligoland. It is the last significant naval battle fought by squadrons of wooden ships, and also the last involving Denmark.
* American general John Sedgwick is shot dead during the Battle of Spotsylvania Court House, shortly after uttering the famous last words: "They couldn't hit an elephant from this distance!"
May 11 - American Civil War - Battle of Yellow Tavern: Confederate General J. E. B. Stuart is mortally wounded at Yellow Tavern, Virginia.
May 12 - American Civil War - Battle of Spotsylvania Court House : Thousands of Union and Confederate soldiers die.
May 13 - American Civil War - Battle of Resaca: The battle begins with Union General Sherman fighting toward Atlanta.
May 18 - Civil War gold hoax: The New York World and the New York Journal of Commerce publish a fake proclamation that President Abraham Lincoln has issued a draft of 400,000 more soldiers.
* American Civil War - Battle of Ware Bottom Church: In the Virginia Bermuda Hundred Campaign, 10,000 troops fight in this Confederate victory.
* Australian bushranger Ben Hall and his gang escape from a shootout with police, after attempting to rob the Bang Bang Hotel in Koorawatha, New South Wales.
June - The United States Sanitary Commission's Sanitary Fair in Philadelphia takes place.
June 5 - American Civil War - Battle of Piedmont: Union forces under General David Hunter defeat a Confederate army at Piedmont, West Virginia, taking nearly 1,000 prisoners.
June 9 - American Civil War - Battle of Petersburg: Union forces under General Grant and troops led by Confederate General Robert E. Lee battle for the last time.
* Battle of Noonday Creek: Confederate troops defeat Union forces, near Kennesaw, Georgia.
* Battle of Brice's Crossroads: Confederate troops under Nathan Bedford Forrest defeat a much larger Union force, led by General Samuel D. Sturgis, in Mississippi.
June 12 - American Civil War - Battle of Cold Harbor: General Ulysses S. Grant pulls his troops from their positions at Cold Harbor, Virginia and moves south.
June 15 - Arlington National Cemetery is established in the United States, when of the grounds of Robert E. Lee's home are officially set aside as a military cemetery, by U.S. Secretary of War Edwin M. Stanton.
July 20 - American Civil War - Battle of Peachtree Creek: Near Atlanta, Confederate forces led by General John Bell Hood unsuccessfully attack Union troops under General William T. Sherman.
July 22 - American Civil War - Battle of Atlanta: Outside of Atlanta, Confederate General Hood leads an unsuccessful attack on Union troops under General Sherman, on Bald Hill.
July 24 - American Civil War - Second Battle of Kernstown: Confederate General Jubal Early defeats Union troops led by General George Crook in an effort to keep the Yankees out of the Shenandoah Valley.
July 28 - American Civil War - Battle of Ezra Church: Confederate troops, led by General Hood, make a third unsuccessful attempt to drive Union forces under General Sherman from Atlanta.
July 29 - American Civil War: Confederate spy Belle Boyd is arrested by Union troops, and detained at the Old Capitol Prison in Washington, D.C.
July 30 - American Civil War - Battle of the Crater: Union forces attempt to break Confederate lines, by exploding a large bomb under their trenches.
August 5 - American Civil War - Battle of Mobile Bay: At Mobile Bay near Mobile, Alabama, Admiral David Farragut leads a Union flotilla through Confederate defenses, and seals one of the last major Southern ports.
August 18 - American Civil War - Battle of Globe Tavern: Forces under Union General Ulysses S. Grant try to cut a vital Confederate supply-line into Petersburg, Virginia, by attacking the Wilmington and Weldon Railroad, forcing the Confederates to use wagons.
*American Civil War: Confederate General Hood evacuates Atlanta, after a 4-month siege mounted by Union General Sherman.
*Charlottetown Conference: Delegates from the Canadian colonies meet, to discuss Canadian Confederation.
September 2 - American Civil War: Union forces under General Sherman enter Atlanta, a day after the Confederate defenders fled the city.
September 5-6 - Bombardment of Shimonoseki: An American, British, Dutch and French alliance engages the powerful feudal Japanese warlord or daimyō Lord Mōri Takachika, of the Chōshū clan, based in Shimonoseki, Japan.
October 12 - Uruguayan War: Forces of the Empire of Brazil invade Uruguay, in support of Venancio Flores' Colorado Party.
October 28 - American Civil War - Second Battle of Fair Oaks: Union forces under General Ulysses S. Grant withdraw from Fair Oaks, Virginia, after failing to breach the Confederate defenses around Richmond, Virginia.
* The Second Schleswig War is concluded. Denmark renounces all claim to Schleswig, Holstein, and Lauenburg, which come under Prussian and Austrian administration.
* Helena, Montana, is founded, after four prospectors discover gold at Last Chance Gulch; it is their last and agreed final attempt for weeks, of trying to find gold in the northern Rockies.
* An annular solar eclipse occurred on October 30, 1864. It was the 42nd solar eclipse of Solar Saros 131.
November 4 - American Civil War - Battle of Johnsonville: At Johnsonville, Tennessee, troops under the command of Confederate General Nathan Bedford Forrest bombard a Union supply base with artillery, and destroy millions of dollars worth of material.
November 22 - American Civil War - Sherman's March to the Sea: Confederate General John Bell Hood invades Tennessee, in an unsuccessful attempt to draw Union General Sherman from Georgia.
November 25 - American Civil War: A group of Confederate operatives, calling themselves the Confederate Army of Manhattan, starts fires in more than 20 locations, in an unsuccessful attempt to burn down New York City.
December 4 - American Civil War - Sherman's March to the Sea: At Waynesboro, Georgia, forces under Union General Judson Kilpatrick prevent troops, led by Confederate General Joseph Wheeler, from interfering with Union General Sherman's campaign of destroying a wide swath of the South, on his march to Savannah; Union forces suffer more than 3 times the casualties as the Confederates, however.
* Syllabus errorum: Pope Pius IX condemns theological liberalism as an error, and claims the supremacy of Roman Catholic Church authority over civil society. He also condemns rationalism and socialism.
December 13 - Paraguayan War: Paraguay formally declares war on the Empire of Brazil, in support of the Uruguayan National Party. The war continues to 1870, with around 300,000 Paraguayan deaths.
December 15-16 - American Civil War - Battle of Nashville: Union forces decisively defeat the Confederate Army of Tennessee.
