Under what legal theory was Mary Surratt hanged as an assassin of President Lincoln in 1865?

Question: Under what legal theory was Mary Surratt hanged as an assassin of President Lincoln in 1865?

Short Answer:
Mary Surratt was convicted as a conspirator who helped plan, provision, and had fore knowledge of the assassination of Abraham Lincoln.

Detailed Answer:
Prior to Lincoln's assasination John Wilkes Booth and David Herold, had stashed two Spencer rifles with ammunition along with other provisions at a Tavern run by John M. Lloyd, 15 miles outside of Washington DC in Surrattesvile. These riffles were considered contraband and would be seized by Union forces if discovered. Lloyd who was hiding them for Booth and Harold at one point was told to burry them so Union forces who randomly searched his tavern would not discover them. On the day of the assassination Mary Surratt rented a carriage and rode 15 miles outside of DC to deliver an additional package from Booth to John M. Lloyd. The package was innocuous, binoculars. But in court John M. Lloyd testified that Mary Surratt told him to have the "shooting irons" (hidden riffles) ready. After Booth assassinated Lincoln latter that same day, he and David Harold visited Lloyd's tavern and picked up the binoculars and riffles as they tried to flee Union pursuit into southern Virginia.

This testimony along with the strong circumstantial evidence was enough to convict Mary Surratt who was hanged as a conspirator. The other evidence was: .

• She was a known southern sympathizer
• She had close ties with many of the conspirators including Booth, Mudd.
• She lived with 4 conspiracy members.
  • George Atzerodt,
  • Lewis Powell
  • Louis J. Weichmann
  • John Surratt (son)
• She attended meetings among the conspiracy members individually and collectively.
• Her son was not only a conspirator he also was tied to espionage for the confederacy and a previous plot to kidnap Lincoln by Booth.

The final straws were that Mary assisted the conspirators by helping stage (binoculars) and make ready provisions(riffles) the day of the assassination. And by telling Lloyd to get the "shooting iron" riffles ready, she demonstrated fore knowledge of the assassination. She knew Booth and Harold would be fleeing through Surrattsville later that evening.

After Lloyd's testimony that was enough to convict her.

The Conspirators were:

• John Wilkes Booth - Shot president Lincoln at Ford's Theater April 15, 1865, killed at Garrett Farm 11 days latter.
• David Herold - accompanied Booth in his escape from DC. Stayed with Booth for 11 days as he evaded Union Pursuit. Was captured at Garrett farm, April 26, 1865 where Booth was killed.
• George Azterodt - Recruited by Booth into the conspiracy, he was assigned to kill Vice President Andrew Johnson, but lost his nerve and stayed in a hotel bar, drinking, instead. Azterodt was executed by hanging in July 1865
• Lewis Powell - Booth assigned Powell to kill Secretary of State William Seward. He entered the Seward home and severely injured Seward, Seward’s son, and a bodyguard. Powell was tried and convicted, and was executed by hanging in July 1865.
• Mary Surratt
• Michael O'Laughlen - Booth’s childhood friend was an ex-Confederate soldier. After he turned himself in to the authorities, he was tried as a conspirator, though his role remained unclear. O’Laughlen was sentenced to life in prison and sent to Fort Jefferson, off of Key West, Florida, where he died of yellow fever in 1867.
• Samuel Arnold - Another long-time friend of Booth, Arnold was not in Washington at the time of the assassination. However, investigators tied Arnold to Booth’s original kidnapping plot. Sentenced to life in prison, Arnold was pardoned by President Andrew Johnson, and survived until 1906, when he died of tuberculosis.
• Samuel Mudd - Prosecutors succeeded in showing that Mudd, a doctor who set Booth’s broken leg during the night of April 14, was well acquainted with Booth before the night of the assassination. He escaped hanging by one vote of the military commission that had been convened to try the conspirators. Like Arnold, Mudd was sentenced to life in prison but pardoned in 1869. He died of pneumonia in 1883.
• Edmund Spangler - Worked at Ford's theatre, knew Booth well and assisted him on April 14 at the theater. He was not connected to the assasination plan beyond that. He was found guilty and sentenced to six years in prison. Pardoned by President Andrew Johnson in 1869, Spangler moved to Maryland, where he remained until his death in 1875.
• John Surratt (not convicted) - A spy for the Confederacy and Mary Surratt's son. Surratt introduced Booth to Herold and Azterodt, and conspired with the others to kidnap the president months prior to the Ford's theatre assassination. John Surratt was not in Washington when the assassination was carried out. Surratt fled the U.S. when he heard news of the crime, and lived in Europe as a fugitive for several years until he was apprehended in Egypt in 1866. Tried by a civilian court in 1867-1868, Surratt was not convicted. He would survive until 1916.
• John M. Lloyd (not tried, nor convicted) - A former DC Policeman who leased a Tavern from Marry Surratt. A Southern sympathizer who held possessions and supplies for John Wilkes Booth and David Herold, including two spensor riffles and ammunition; which they picked up as they fled Washington after Lincoln's assasination. His testimony at trial was among the most damaging presented against Mary Surratt.