Political map of North America & the Caribbean 4 July 1863 (Vicksburg and Gettysburg): After a stunning victory at Chancellorsville (Battle of Chancellorsville), Lee (Robert E. Lee)'s Army of Northern Virginia struck north into Pennsylvania only to be decisively defeated at Gettysburg (Battle of Gettysburg). Meanwhile Union forces captured Vicksburg (Siege of Vicksburg), completing their control of the Mississippi River and physically splitting the Confederacy.