Short answers, pulled from the story.
The impeachment of Andrew Johnson was initiated by the United States House of Representatives on the 24th of February 1868. Eleven articles were adopted by the House on March 2 and 3, 1868 to specify alleged high crimes and misdemeanors.
Andrew Johnson violated the Tenure of Office Act passed in March 1867 over his veto. The primary charge against him was that he acted to remove Edwin Stanton from Secretary of War position without Senate consent and replaced him with Brevet Major General Lorenzo Thomas as secretary of war ad interim.
Conviction required a two-thirds majority vote which equated to 36 votes out of 54 senators present. On the 16th of May 1868 the Senate voted 35 to 19 favoring conviction falling one vote short of the necessary threshold.
Chief Justice Salmon P. Chase presided over the proceedings of the Senate trial beginning the 5th of March 1868. Chase maintained deciding certain procedural questions alone was his prerogative though the Senate challenged several rulings regarding his authority.
Seven Republican senators including Lyman Trumbull believed convicting Johnson would destroy the balance between branches of government and reduce presidential power to mere disagreement over policies. They argued impeachment should be reserved for actual crimes rather than political differences or policy disagreements.