Upon the outbreak of war in 1775–1776 between Patriots who rebelled against the colonial administration and Loyalists who continued to support British government, de Peyster chose the Monarchist side.

[1] This became dangerous duty in the rebel colony of South Carolina, where de Peyster served under and tried to assist his regimental commander, Major Patrick Ferguson.

Major Ferguson had led the regiment into the Upcountry of South Carolina, where he and his men were badly outnumbered, and into the decisive Battle of Kings Mountain on October 7, 1780.

[2] Following the decisive defeat of the British and Loyalist forces at the Battle of Yorktown in 1781, de Peyster was allowed to return to the royal garrison in his home city of New York.

[1] In consideration of his services to the Crown, de Peyster was granted lands and appointed to a variety of official posts.