Following the Union Army's rout at the First Battle of Bull Run (Manassas) in late July 1861, the Union Army constructed Fort Tillinghouse in August of that year. The lunette was part of the Arlington Line and tied into Fort Craig approximately to the south and Fort Cass approximately to the north. Along with Forts Cass, Woodbury, Morton, and Strong, the fortification covered approaches to the Long Bridge and the Aqueduct Bridge (near the modern Key Bridge).