With the opening of the Southern theater in the American Revolutionary War, Guerard enlisted in the militia and participated in the siege of Charleston by the British in 1780. The colonists surrendered Charlestown on May 12, 1780 and Guerard became a prisoner, although he was paroled.
Guerard joined the militia when the Southern theater of the American Revolutionary War began, participated in the siege of Charleston by the British in 1780, and became a prisoner after Charlestown surrendered to the British on May 12, 1780. He was later released from captivity.