An early incident revealed Boyd's commitment and courage and ironically brought her praise from Federal officers. After Boyd's father left to join Thomas J. "Stonewall" Jackson's infantry, Union troops invaded Martinsburg on 3 July 1861. The next day, Union soldiers forcibly entered the Boyd home and prepared to hoist the Federal flag and remove the Confederate flags in Belle's room and outside the house. Mary Boyd's protests provoked an officer to address her "in language as offensive as it is possible to conceive," as Belle recalled in her 1865 memoir Belle Boyd in Camp and Prison. Infuriated, the seventeen-year-old Belle shot and killed the officer, for which she gained the admiration of the Federal command, who said she had "done perfectly right." Becoming something of a celebrity to Confederates and Unionists alike, Belle became acquainted with many young Union officers posted to protect her home, and she passed on to the Confederates information that the Union officers inadvertently revealed to her, thus becoming an unofficial Confederate spy. Boyd gathered information by flirting with Union officers, listening carefully to what they let drop, and by watching troop movements.
Lyde Cullen Sizer, "Boyd, Belle," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-01228.html.