When soldiers of the Confederate States Army invaded Pennsylvania in the summer of 1863, Ten Eyck advocated immediate creation of a Burlington County militia unit to take part in Pennsylvania's defense. He enlisted as a private in order to set an example for those in attendance, and they called the unit they created the "Ten Eyck Guards" in his honor. He marched to Pennsylvania with the company, and once the Confederates had retreated, he completed his term of service upon the return of the Ten Eyck Guards to Burlington County.
