United States representative Daniel Sickles, after learning of an affair between his wife and Attorney General Philip Barton Key II, murders him in Washington, D.C.

The United States House of Representatives is a chamber of the bicameral United States Congress; it is the lower house, with the U.S. Senate being the upper house. Together, the House and Senate have the authority under Article One of the U.S. Constitution in enumerated matters to pass or defeat federal government legislation, known as bills. Those that are also passed by the Senate are sent to the president for signature or veto. The House's exclusive powers include initiating all revenue bills, impeaching federal officers, and electing the president if no candidate receives a majority of votes in the Electoral College.

Daniel Edgar Sickles was an American politician, Civil War veteran, and diplomat. He served in the U.S. House of Representatives both before and after the war.

Philip Barton Key II was an American lawyer who served as U.S. Attorney for the District of Columbia. He is most famous for his public affair with Teresa Bagioli Sickles, and his eventual murder at the hands of her husband, Congressman Daniel Sickles of New York. Sickles defended himself by adopting a defense of temporary insanity, the first time the defense had been successfully used in the United States.

The trial of Daniel Sickles was an American criminal trial. It was the first time that a defense of "temporary insanity" was used in American law, and it was one of the most controversial trials of the 19th century. Daniel Sickles was a U.S. representative from the State of New York, and Philip Barton Key II was the Attorney General for the District of Columbia. On 27 February 1859, Sickles, after learning that Key was having an affair with his wife, Teresa, approached Key in Lafayette Square and shot and killed him. Sickles turned himself in and was charged with murder. Sickles' defense team, which included lawyers James T. Brady and Edwin Stanton, argued that Sickles had been "temporarily insane" at the time of the murder, and therefore was not guilty. The trial was the subject of extensive media coverage, which created its own controversies and destroyed Teresa's reputation. The jury acquitted Sickles after deliberating for 70 minutes.