- May 1 -8] Battle at Alexandria, Louisiana (Red River Campaign)
- May 1 Atlanta campaign in Georgia begins during US Civil War
- May 3 3rd day in Battle at Alexandria Louisiana: Confederate assault
- May 4 -16] actions at Drewry's Bluff, Virginia
Attack at Rappahannock River
May 4 General Ulysses S. Grant's Union Army at Potomac attacks Robert E. Lee's Confederates at Rappahannock River
- May 5 Atlanta Campaign: 5 days fighting begins at Rocky Face Ridge
- May 5 Battle between Confederate & Union ships at mouth of Roanoke
- May 5 Battle of Wilderness, Virginia (Germanna Ford, Wilderness Tavern)
- May 5 Campaign in Northern Georgia - Chattanooga to Atlanta
- May 6 Battle of Port Walthall Junction, fought in Virginia begins (Battle of Port Walthall Junction), Union victory (US Civil War)
Battle of Wilderness
May 6 Battle of Wilderness, fought in Virginia, Confederate General James Longstreet seriously injured (Overland Campaign), inconclusive result (US Civil War)
Sherman Begins Atlanta Advance
May 6 US Civil War: Union Army General Sherman begins advance to Atlanta, Georgia, during the Atlanta Campaign
- May 7 Battle of Wilderness ends inconclusively in Virginia, Union losses 17,666; Confederates 7,500
- May 7 Skirmish at Port Walthall Junction, Virginia (Drewry's Bluff)
- May 8 Actions at Stony Creek/Nottoway bridge, Virginia (Drewry's Bluff)
- May 8 Atlanta Campaign: Severe fighting near Dalton
- May 8 Battle of Spotsylvania Court House begins
- May 9 -20] Skirmish at Ware Bottom Church, Virginia
- May 9 Battle of Cloyd's Mt, & Swift Creek, Virginia (Drewery's Bluff, Fort Darling)
- May 9 Naval Battle of Helgoland, last naval battle fought by squadrons of wooden ships, inconclusive between Danish and a Austro-Prussian alliance, but ends blockade of German ports
- May 10 Battles at Spotsylvania Court House, Virginia
- May 10 Skirmish at Ny River, Virginia
- May 11 Battle of Yellow Tavern, Virginia (Sheridan's Raid, South Anna Bridge)
Battle of Yellow Tavern
May 11 Confederate General J.E.B. Stuart is mortally wounded at the Battle of Yellow Tavern in Henrico County, Virginia
- May 12 Battle of Drewry's Bluff, fought in Chesterfield County, Virginia (Peninsula Campaign), Confederate victory (US Civil War)
- May 12 Battle of Spotsylvania, fought near Courthouse, Virginia (Overland Campaign), inconclusive result (US Civil War)
- May 12 Battle of Todd's Tavern, cavalry battle fought near Todds Tavern, Virginia (Sheridan's Raid), inconclusive result (US Civil War)
- May 12 US Union colonel Emory Upton (24) promoted to brigadier general
- May 13 -16] Atlanta Campaign: Battle of Resaca, Georgia
- May 15 Battle of New Market, Virginia
- May 15 Skirmish at Marksville (Avoyelles) (Red River Campaign)
- May 16 Atlanta Campaign: Battle of Resaca, ends (since May 13)
- May 16 Battle of Bermuda Hundred, Virginia
- May 16 Last battles at Drewry's Bluff, Virginia (6,666 casualties)
- May 17 Battle of Adairsville Georgia, Union forces Confederates to retreat
- May 18 Battle of Yellow Bayou, Louisiana (Bayou de Glaize, Old Oaks)
- May 19 Battle of Port Walthall Junction, Virginia (Bermuda Hundred)
- May 19 Last engagement in series of battles known as Spotsylvania
- May 19 Skirmish at Cassville, Georgia
- May 20 Battle at Ware Bottom Church, Virginia, 1,400 killed or injured
- May 21 Battle of Spotsylvania Court House of the US Civil War ends inclusively with an estimated 32,000 casualties on both sides
- May 21 General David Hunter takes command of Department of West Virginia
- May 21 Russia declares an end to the Russian-Circassian War and many Circassians are forced into exile. The day is designated to be the Circassian Day of Mourning.
- May 22 Battle of North Anna River, Virginia (Totopotamy River, Haw's Shop, Hanovertown)
- May 23 Battle of North Anna, Virginia, 1st of 3 days of fighting
- May 25 US Civil War: Battle of New Hope Church, Georgia: Major William T. Sherman's Union forces launch attack but suffers heavy losses and entrenchment on both sides [1]
- May 26 -30] Skirmish along the Totopotomoy Creek, Virginia
- May 26 Battle of Dallas, begins near Paulding County, Georgia (Atlanta Campaign), Union victory (US Civil War)
- May 26 Territory of Montana formed
- May 27 Skirmish at Salem Church (Haw's Shop), Virginia
- May 29 Mexican Emperor Maximilian arrives at Vera Cruz
- May 30 Battle of Bethesda Church [Totopotomoy Creek], cavalry battle fought in Hanover County, Virginia, inconclusive result (US Civil War)
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