After months of training, Monroe and 700 Virginia infantrymen were called north to serve in the New York and New Jersey campaign. Shortly after the Virginians arrived, George Washington led the army in a retreat from New York City into New Jersey and then across the Delaware River into Pennsylvania. In late December, Monroe took part in a surprise attack on a Hessian encampment at the Battle of Trenton. Though the attack was successful, Monroe suffered a severed artery in the battle and nearly died. In the aftermath, Washington cited Monroe and William Washington for their bravery, and promoted Monroe to captain. After his wounds healed, Monroe returned to Virginia to recruit his own company of soldiers.[5] His participation in the battle was memorialized in John Trumbull's painting The Capture of the Hessians at Trenton, December 26, 1776 as well as Emanuel Leutze's 1851 Washington Crossing the Delaware.[6]

The Capture of the Hessians at Trenton, December 26, 1776, by John Trumbull, showing Captain William Washington, with a wounded hand, on the right and Lt. Monroe, severely wounded and helped by Dr. John Riker, left of center, behind the mortally wounded Hessian Colonel Johann Gottlieb Rall. 