March: First Conscription Act passed. Produces riots in predominantly Irish working class neighborhoods in New York.
April 24: Lieber Code is adopted by the Union War department as General Order 100.
May: Vicksburg Campaign.
May 1: Confederate Congress passes a joint resolution permitting the summary execution of white Union officers commanding black Union troops as having committed the crime of "inciting servole insurrection."
May 4: Battle of Chancellorsville.
May 5: Battle of Tupelo.
May 10: Stonewall Jackson dies in friendly fire at Chancellorsville.
May 25: Prisoner exchange of officers between the Union and Confederacy is terminated.