Political sex scandals are as old as politics and so commonplace that even in the internet age we can barely keep up with developments in the latest salacious story. One such scandal, however, holds a more prominent place than most in American legal history. This one had it all: an aggrieved husband, a remorseful wife, and a handsome lover. This was not just a sex scandal, but also a murder case. The killer was a sitting U.S. Congressman, and his victim the son of The Star-Spangled Banner’s author. This case gave us not only juicy headlines, but the first-ever American use of the temporary insanity defense.

In 1859, Daniel Edgar Sickles was a Democrat representing New York’s Third Congressional District in the U.S. House of Representatives. He was a well-known ladies man and serial adulterer whose wife, Theresa Bagioli Sickles, was just fifteen years old – and pregnant – when he married her in 1852. (Sickles was thirty-three when they married.) Philip Barton Key II, a friend of Congressman Sickles, was the United States Attorney for the District of Columbia and the son of Francis Scott Key, author of the poem that eventually became the national anthem. Key was called the “handsomest man in Washington,” and Sickles often asked his friend to escort his wife to social events the Congressman could not attend. The relationship between Key and Mrs. Sickles turned romantic, and in early 1859 Daniel Sickles received an anonymous letter informing him of his wife’s infidelity. When confronted, Theresa Sickles confessed everything, including Key’s habit of standing across Lafayette Square and waving a handkerchief at the Sickles home to signal Theresa to meet him. The Congressman forced his wife to write a long and detailed letter of confession.

On February 27, 1859, Sickles looked out the window and saw Key waving his handkerchief. Enraged, the Congressman stormed outside and approached Key, shouting, “Key, you scoundrel, you have dishonored my home. You must die!” Sickles pulled a pistol and shot Key several times. When the wounded man pleaded for his life, Sickles drew a second pistol and shot him in the chest. He then fled to a friend’s home, where he surrendered to police a few hours later. (The friend, by the way, was Jeremiah Black, Attorney General of the United States.)

Sickles was charged with murder and taken to jail to await trial. During his detention, he received visits from numerous Congressional colleagues and government officials; President James Buchanan sent a supportive personal letter. Public sentiment was strongly in his favor as well, especially after details of his wife’s confession became known. Sickles was portrayed as a wronged, grieving husband who had only sought to restore his honorable name. As his trial approached, his legal team – which included future Secretary of War Edwin M. Stanton – proposed a defense never before tried in the United States: not guilty due to temporary insanity.

The modern insanity defense came out of an 1843 case in England. Daniel M’Naghten, thinking himself the target of a conspiracy led by the Vatican and British Prime Minister Robert Peel, tried to murder Peel but killed the Prime Minister’s secretary instead. When tried, M’Naghten was ruled delusional and the jury acquitted him of murder. Following public outrage, the House of Lords convened a panel of judges and posed numerous hypothetical questions to them about insanity. The panel’s answers and recommendations became known as the M’Naghten Rules and state that a defendant can use an insanity defense if “at the time of the commission of the acts…the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality of the wrongfulness of his acts.” In other words, could the defendant differentiate between right and wrong?

At trial, Edwin Stanton argued that learning of Theresa’s affair drove Congressman Sickles temporarily insane and that he was not in his right mind when he murdered his wife’s lover. Therefore, Sickles could not be held liable for his actions. Newspaper accounts provided graphic details of Mrs. Sickles’ affair with Key, which very likely came from the confession her husband forced her to write. These articles often applauded Sickles for his actions, and the jury quickly acquitted him of murder. After being released from jail, Sickles immediately destroyed public goodwill by forgiving his wife and returning to her. His reputation ruined and his prospects for reelection dim, he returned to New York in disgrace. He and Theresa remained married until she died of tuberculosis in 1867.

Fate soon intervened to bring Sickles greater fame than he acquired by shooting Key. The Civil War began in 1861, and Sickles obtained a Union army commission. By the July 1863 battle of Gettysburg, he was a Major General commanding the Army of the Potomac’s Third Corps. After violating his orders and moving his entire corps forward on the battle’s second day, General Sickles was wounded by an artillery shell and had his right leg amputated. He promptly sent the leg to Washington, where he supposedly visited it every July 2 for years to come. He also argued for the rest of his life that his unauthorized movement saved the Union army at Gettysburg, and he was awarded the Medal of Honor on October 30, 1897. Sickles died at age ninety-four on May 3, 1914, and is interred in Arlington National Cemetery.

Today, many more people know about Daniel Sickles for his role at Gettysburg than his dubious distinction as the first American to dodge a murder conviction due to temporary insanity. Though the temporary insanity defense is rarely attempted and even more rarely successful, it is certainly worth noting when it was first used. Though Sickles likely would have preferred to be remembered for his military exploits, just knowing that we’re still talking about him at all would probably make him very happy.