 Booth and a group of co @-@ conspirators originally plotted to kidnap Lincoln but later planned to kill him , Vice President Andrew Johnson , and Secretary of State William H. Seward in a bid to help the Confederacy 's cause . Although Robert E. Lee 's Army of Northern Virginia had surrendered four days earlier , Booth believed the American Civil War was not yet over because Confederate General Joseph E. Johnston 's army was still fighting the Union Army . 
