After getting the consent of his master, he volunteered in 1781 to join the American army under the Marquis de Lafayette. The Marquis used him as a spy. Armistead posed as a runaway slave to the British. He joined the camp of Brigadier General Benedict Arnold the turncoat who was leading some British forces in the area. Pretending to be a spy for the British, Armistead gained Arnold's trust. Arnold used him to guide British troops through local roads. ""The ex-slave, who later renamed himself James Armistead Lafayette in the general's honor, served as a double agent against the british under the avowedly antislavery Lafayette."" 