1-4 – The Army of Northern Virginia wins a decisive battle at Chancellorsville, Va.
1-2 – Union forces under Maj. Gen. Ulysses S. Grant drive Rebels from Port Gibson in Mississippi, opening a path to Vicksburg.
2 – Confederate Lt. Gen. Thomas J. “Stonewall” Jackson is mortally wounded by friendly fire at Chancellorsville.
5 – On orders of Maj. Gen. Ambrose Burnside, Federal troops arrest former Congressman Clement Vallandigham, leader of the Peace Democrats (Copperheads), in Dayton, Ohio. Tried May 6 by a military commission in Cincinnati, he is convicted of expressing treasonable sympathies and will be banished to the Confederacy on May 25.
7 – Confederate Maj. Gen. Earl Van Dorn is murdered in Spring Hill, Tenn., by the jealous husband of an alleged lover.
10 – Stonewall Jackson dies from complications of his wounds at a home near Guinea Station, south of Fredericksburg, Va.
11 – Secretary of the Treasury Salmon P. Chase threatens to resign from Abraham Lincoln’s Cabinet because of a disagreement over an appointment.
16 – Grant’s Army of the Tennessee defeats Lt. Gen. John C. Pemberton’s forces at Champion Hill, 20 miles east of Vicksburg. The 43-day Siege of Vicksburg begins two days later.
22 – The U.S. Department of War establishes the Bureau of Colored Troops to manage the overwhelming number of black recruits.
28 – The all-black 54th Massachusetts leaves Boston for Hilton Head, S.C.
1 – Burnside orders suppression of the anti-administration Chicago Times. Chicago Mayor F.C. Sherman and other city residents ask Lincoln to rescind the order. Lincoln tells Secretary of War Edwin Stanton to revoke the order on May 4.
2 – Confederate President Jefferson Davis orders Vallandigham to Wilmington, N.C., and kept under guard as an “alien enemy.” Vallandigham will eventually be sent to Canada.
3 – New York Mayor Fernando Wood leads local Democrats in a meeting to urge peace.
Robert E. Lee begins sending troops west from Fredericksburg in the first stages of what will become the Gettysburg Campaign.
9 – At the Battle of Brandy Station, Union forces led by Maj. Gen. Alfred Pleasonton surprise Rebels under Maj. Gen. Jeb Stuart in the largest cavalry battle on American soil. Stuart manages to hold the field.
11 – Peace Democrats nominate Vallandigham in absentia for governor of Ohio.
20 – West Virginia officially enters the Union as the 35th state.
23 – The Tullahoma Campaign begins in Tennessee as Federals under Maj. Gen. William Rosecrans confront Confederates under General Braxton Bragg.
24 – The Army of Northern Virginia begins crossing the Potomac en route to Pennsylvania.
27 – Lincoln relieves Maj. Gen. Joseph Hooker as commander of the Army of the Potomac, replacing him with Maj. Gen. George Gordon Meade.
28 – Confederate Maj. Gen. Jubal Early seizes York, Pa. Lee orders his forces to concentrate near Gettysburg.