He was impeached for violating the Tenure of Office Act, which
forbid him to dismiss a cabinet member without congressional
approval. The Senate trial lasted 11 and a half weeks. On the major
charges, the Senate voted 35-19 for conviction, one vote short of
the 2/3s vote required for removal from office.
Citation:
Editors. "Impeachment of Andrew Johnson".
The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=10965, accessed 19 May 2013.]