Monocacy (mənŏk`əsē), river, c.60 mi (100 km) long, rising in S Pa., and flowing S across Md. to join the Potomac River near Frederick, Md. On its banks, just E of Frederick, the Civil War battle of Monocacy was fought on July 9, 1864. Although the Union forces under Gen. Lew Wallace were defeated, they delayed the Confederate forces under Gen. J. A. Early long enough to give Grant time to dispatch troops to defend Washington and drive Early back into Virginia.