There are several competing theories. One is that Booth and Johnson were friends. A 1907 book by Hamilton Howard called "Civil War Echoes" claims that they both had mistresses who were sisters. Another theory is that Johnson was from Tennessee and did not agree with Lincoln's push to keep the union together. The problem with all the theories is that one of Booth's confederates (no pun intended), George Atzerodt, was supposed to assassinate Johnson and Lewis Powell was to kill William Seward, to completely put the country in a confusion from lack of leadership.