Mayfield Fort was one of twelve Confederate earthwork defenses built in the area to protect the junction of the Orange and Alexandria and Manassas Gap Railroads, and it is the only one of the original twelve that remains. Construction began in 1861 by order of General Robert E. Lee and was supervised by troops under the command of General P.G.T. Beauregard. After Confederate troops withdrew from the area in March 1862, Union forces occupied and garrisoned the fort until late 1864. The fort also stood near an early Civil War signaling event: on the morning of July 21, 1861, CSA Captain Edward Porter Alexander, stationed on nearby Signal Hill, saw a column of Union troops marching toward Sudley Springs and sent the message "Look out for your left is turned" to Confederate forces at Stone Bridge, the first use of the wig-wag signaling system during the war and an event probably seen by the men at Mayfield Fort.
