While in Philadelphia, Rev. Rosbrugh was given a commission as a Chaplain. Captain John Hays took his place as company commander. A chaplain at that time held the rank of major and pay of thirty-three and a half dollars per month. Presbyterian ministers, however, were particularly hated by the British and if captured suffered cruel treatment. Rev. Rosbrugh’s company joined the rest of the Northampton county militia in camp. Rosbrugh also met with his brother-in-law, John Ralston, a member of the Second Continental Congress. The 3rd Battalion was under the command of General Israel Putnam. He had orders to cross the Delaware on the 25th of December in support of Washington’s surprise attack on Trenton. However General Putnam decided not to cross due to weather conditions. So Rev. Rosbrugh’s battalion remained in Philadelphia. After General Washington crossed back to Pennsylvania he decided to take Trenton a second time. This time the 3rd Northampton militia was included. The battle was called the second battle of Trenton or the battle of Assinpink creek.