On May 1st, Union troops under Benjamin Butler began entering the strategically vital city of New Orleans.
On May 3rd, Confederate General Joseph E. Johnston suddenly withdrew from the Warwick Line in the Battle of Yorktown. The sudden retreat ceded the battle to Union forces under McClellan.
The Battle of Williamsburg was fought on May 5th. This was the first major clash of the Peninsula Campaign, involving around 41,000 Federals and some 32,000 Confederates. Joseph Hooker's Union division encountered the rear guard of Joseph Johnston's Rebel troops fleeing the Battle of Yorktown. This rear guard, Jeb Stuart's cavalry, skirmished with Stoneman's Union horse troopers who had been sent by McClellan after their unexpected withdrawal from Yorktown. Johnston, trying to buy time for his retreat, detached troops to man a large earthen fortification called Fort Magruder, straddling the Yorktown-Williamsburg road. Hooker assaulted the fort, but was repulsed by counterattacks by Confederate General James Longstreet. Hooker was expecting help from William "Baldy" Smith, but Smith, fearing a Confederate attack on his position, held up a little over a mile away. Longstreet's attacks pushed Hooker's troops back. A Union brass band playing "Yankee Doodle" managed to slow the retreat until General Phil Kearny came up with his division. Kearny displayed characteristic dash and daring, riding out in front of the line and urging the Union troops to the attack with a wave of his sword. The Union troops pushed the Confederates back. Winfield Hancock's Union division began an artillery bombardment of Longstreet's left flank, disobeying orders to fall back. After a failed attack by Jubal Early, Hancock's men executed a superb bayonet charge, rolling up the Confederate line. The battle was trumpeted as a major victory by the Northern press, but in reality, Johnston's fight proved to be a delaying action which allowed the bulk of the Confederate army to retreat to Richmond.
On May 7th, they clashed again in the Battle of Eltham's Landing. This time Union troops under William Franklin tried to attack the Barhamsville Road, attempting to disrupt the Confederate retreate from Williamsburg. The Rebels successfully resisted the attack and continued their retreat.
May 8th saw the Battle of McDowell in the Shenandoah. Stonewall Jackson pushed Union troops under Schenk and Milroy off of a strategic ridge after a fierce and bloody fight, setting the stage for Jackson's successful Valley Campaign.
Union General David Hunter freed the slaves in South Carolina, Florida, and Georgia on May 9th. On that same day, Confederate troops destroyed military facilities at Norfolk before continuing their retreat down the peninsula.
On May 10th, Confederate ships clashed with a Union squadron consisting of several ironclads and mortar boats. The Confederate ships defeated the Union force, actually sinking two of the ironclads, but were unable to prevent the Union navy from proceeding down river towards Memphis, Tennessee.
Also on May 10th, Confederates destroyed the naval base at Pensacola, Florida.
On May 11th, Confederate sailors, unable to get the CSS Virginia (aka Merrimack) ironclad out of Norfolk harbor in the face of approaching Union troops, scuttled the ship.
On May 15th, The Union navy sailed upriver to Richmond to test the city's defenses. In the Battle of Fort Drewrey, the ships encountered deadly accurate fire and submerged defenses and were forced to retreat.
Also on May 15th Benjamin Butler issued an order that any New Orleans woman who insulted the Union troops would be treated as a prostitute. It was this order that birthed Butler's nickname, "The Beast of New Orleans."
On May 19th, President Lincoln rescinded David Hunter's emancipation order and used the opportunity to call for a gradual emancipation of the slaves.
On May 23rd, the Battle of Front Royal was fought. Stonewall Jackson demonstrated his brilliance as a tactician, using the topography of the area around Front Royal, Virginia to divide and defeat his Union foes at little cost to himself. The Union troops, faced with an impossible situation despite having a numerical superiority, were forced to withdraw. Union losses were 776. Confederate losses were only 36. This battle is also noteworthy because for the only time in the Civil War, troops from the same state faced each other in battle. The 1st Maryland Regiment of the Union and the 1st Maryland Regiment of the Confederates fought each other, with Confederate Captain William Goldsborough capturing his brother, Union Captain Charles Goldsborough during the fight.
Confederate troops under Stonewall Jackson continued their march, attacking Union forces under Nataniel Banks at Winchester on May 25th. They attacked and beat Union forces at Middletown and Newtown, capturing much of the Union Army's supplies. Maneuvering with brilliance and unprecedented speed, Jackson's troops forced Banks' forces out of Winchester and eventually across the Potomac River and into Maryland. This was a major victory, in that Union forces were withdrawn from the Peninsula Campaign on Richmond to be redeployed against Jackson. In fact, on May 27th, a Union victory at Hanover Courthouse bacame moot as those forces were recalled and marched across the state to the Shenandoah.
On May 30th Rebel forces evacuated Corinth, Mississippi in the face of Henry Halleck's glacial advance of 26 miles in 20 days.
May 31st saw the Battle of Seven Pines as part of the Peninsula Campaign. It was an inconclusive fight (both sides claimed victory, but in terms of the human cost, was the most expensive battle, second only to Shiloh. It could be called a strategic victory for the South, since the battle marked the end of the Union offensive, which reached as far as the outskirts of Richmond.