After the assassination of President Abraham Lincoln in April 1865, Vice President Andrew Johnson assumed the U.S. presidency. As Johnson was implementing his plans for Reconstruction after the Civil War, he encountered opposition from Republicans in Congress who did not agree with his lenient position toward southern radicals. As time passed, Johnson and Congress clashed on various acts related to Reconstruction. Johnson vetoed many bills and had many vetoes overridden by a hostile Congress. His most controversial action, however, was the removal of Secretary of War Edwin Stanton. Congress asserted that Johnson had disobeyed the Tenure of Office Act and, because of this and related crimes and misdemeanors, was therefore subject to impeachment.