According to the History Channel, President Andrew Johnson attempted to fire Secretary of War Edwin Stanton on multiple occasions because Stanton opposed Johnson's more lenient attitude toward the South during Reconstruction. Johnson wanted to readmit states from the Confederacy without any guarantees of civil rights for the freed slaves, and Stanton strongly opposed this policy and favored much harsher treatment of the former rebellious states.
In 1867, President Johnson decided that Stanton's opposition was crippling his presidency, so he attempted to remove Stanton from office. Stanton refused to leave, claiming that the Tenure of Office Act prevented his removal. In 1868, Johnson suspended him and appointed Ulysses S. Grant as his replacement, but the Senate overruled President Johnson and Stanton continued in his position. Johnson tried a third time to remove him from office, appointing General Lorenzo Thomas as a replacement, but Congress again backed Stanton.
Radical Republican forces in Congress began an impeachment trial against Johnson over his continued attacks on Stanton, but they were unable to gather enough votes in the Senate to remove the president from office. In the end, Edwin Stanton resigned his post on May 26, 1868. Stanton was eventually appointed to the Supreme Court by Ulysses S. Grant in 1869, but he died before he could take office.