Source: http://spotlights.fold3.com/2013/02/25/a-case-of-temporary-insanity/

Daniel Sickles, although perhaps best known as a political Civil War general whose disobedience at Gettysburg got his troops killed, is also known for a scandal before the war in which he killed his wife’s lover.

Sickles was a former lawyer of the Tammany Hall political machine who had become a Democratic representative for New York in the House of Representatives. Sickles was a womanizer and had many affairs, most famously with courtesan Fanny White, whom he took with him on his travels to England. At age 33 he courted scandal by marrying Teresa Bagioli—who was 15 and pregnant.

In 1859, when his wife was 23, Sickles discovered that she had been having an affair with 40-year-old Philip Barton Key, a family friend who was also a US district attorney and the son of composer Francis Scott Key. Sickles had apparently been ignorant of the affair until he received an anonymous letter informing him of his wife’s actions. Sickles confronted his wife and she confessed.

The next day, February 27, he saw Key walking by the house, and Sickles ran out shouting, “Key, you scoundrel, you have dishonored my home—you must die!” Sickles cornered Key, assaulting him and shooting him multiple times. Finally, Sickles shot Key point blank in the chest, killing him.

As there had been multiple witnesses, there was no doubt of Sickles’s guilt, but when his case came to trial, his lawyer (Edwin Stanton, later Lincoln’s secretary of war) blamed temporary insanity—the first time that defense had been used in trial. The trial received much attention, and the public—as well as the jury—sided with Sickles, believing that his actions and insanity were totally justified given the situation, and he was acquitted.

Interestingly, public opinion stayed on Sickles side until a few months later, when he reconciled with his wife. Having completely vilified Teresa, the public couldn’t forgive Sickles for returning to her, and they turned against him. It seemed like his political career would never recover, but then the war started and Sickles became a Union general—giving him a chance to start over.

Daniel Edgar Sickles