On 1 January 1777, Campfield joined the Continental Army as a surgeon appointed to
Spencer's Additional Continental Regiment of the Continental Army. Following the Continental Army's extended campaign and victories at Trenton and Princeton in December 1776 and early January 1777, the army marched to Morristown for its winter encampment, arriving on 6 January 1777. Upon their arrival, the army suffered a smallpox outbreak. Campfield provided medical support to help suppress the outbreak and assisted in implementing an inoculation program ordered by General George Washington. He participated in the Battles of Short Hills in June 1777, Brandywine in September 1777, and Germantown in October 1777. He is listed on the muster rolls during several months of the Valley Forge encampment during the winter of 1777-1778 and was in command during his assignment. In 1779, General George Washington organized an expedition into western New York and western Pennsylvania to respond to attacks by Iroquois warriors and their allied Tory rangers. Campfield joined this expedition as a surgeon attached to the regiment, keeping a detailed diary of his experiences and observations of the natural environment, Native American life, and the hardships of wilderness travel with the army. Watching the burning of one Native American village after another, Campfield recognized and was bothered by the apparent genocide, though oddly he blamed it on the victims. On 11 August 1779 he wrote, "I very heartily wish these rusticks may be reduced to reason, by the approach of this army, without their suffering the extremes of war; there is something so cruel, in destroying the habitations of any people (however mean they may be, being their all) that I might say the prospect hurts my feelings." Almost one hundred years later, his diary was saved from the trash by a local historian Edmund D. Halsey, who then donated it to the
New Jersey Historical Society in 1873 and arranged for it to be published in the proceedings of the society the same year. Only a few weeks after Campfield returned home, in December 1779 Washington and the Continental Army returned to Morristown for a second winter encampment. In June 1780, Dr. Campfield served in the Battle of Springfield. Spencer's Regiment was dissolved on 1 January 1781, and after a few months break, Campfield rejoined the army as a surgeon to the
2nd Continental Light Dragoons, serving under his old teacher William Burnet. He continued to serve in this unit until the end of the war, and mustered out of the army at
Danbury, Connecticut on 15 June 1783, returned to Morristown, and resumed his medical practice. == Later life and career ==