- Aug 4 US government collects its 1st income tax
- Aug 5 Battle of Baton Rouge, Louisiana, Union forces prevent Confederate attempts to capture the city
- Aug 6 Confederate ironclad "Arkansas" is badly damaged in Union attack
- Aug 9 Battle of Cedar Mountain [Slaughter's Mountain], fought in Culpeper County, Virginia, Confederate victory (US Civil War)
Beatrice et Benedict
Aug 9 Hector Berlioz's opera "Beatrice et Benedict" premieres in Baden-Baden, Germany
- Aug 10 Battle of Nueces River TX
Morgan Captures Gallatin
Aug 12 Confederate General John Hunt Morgan and his raiders capture Gallatin, Texas
Lincoln Meets Black Leaders
Aug 14 Abraham Lincoln receives the first group of African Americans to confer with a US president
- Aug 17 Confederate troops under Edmund Kirby Smith enter Kentucky
Lee's Adjutant Captured
Aug 18 General Lee's adjutant Major Stuart captured [1]
Stuarts Raid of Revenge
Aug 22 US Civil War: Confederate Major General J.E.B. Stuart, in a retaliatory raid, captures the dress coat of Union General John Pope at Catlett Station, Virginia [1]
- Aug 23 Skirmish at Big Hill, Kentucky (2 Federal regiments)
- Aug 25 US Secretary of War authorizes Gen Rufus Saxton to arm 5,000 slaves
- Aug 28 Battle of Groveton, Virginia (Manassas Plains) [->AUG 19]
- Aug 28 Battle of Thoroughfare Gap, Virginia
- Aug 28 Belle Boyd released from Old Capital Prison in Washington, D.C.
- Aug 29 Battle of Aspromonte; Italian royal forces defeat rebels
Second Battle of Bull Run
Aug 29 Second Battle of Bull Run, fought in Manassas, Virginia begins, Confederate victory (US Civil War)
- Aug 29 US Bureau of Engraving & Printing begins operation
United States One Dollar Bill
Aug 29 US Department of the Treasury forms division now known as US Bureau of Engraving and Printing to print newly issued paper currency
- Aug 30 Battle of Altamont-Confederates beat Union forces in Tennessee
- Aug 30 Second Battle of Bull Run in Manassas, Virginia, ends with a Confederate victory over Union forces