Arbuthnot and Ambrister incident
The Arbuthnot and Ambrister incident occurred in April 1818 during the First Seminole War when American general Andrew Jackson invaded Spanish Florida and his troops detained two British subjects, Alexander Arbuthnot and Robert Ambrister. They were later tried by court-martial, found guilty of aiding the Native Americans against the United States, and executed.
Source: Wikipedia — Arbuthnot and Ambrister incident (CC BY-SA 4.0)