Even though many regular army officers from the south had been opposed to secession, the Confederate attack on Fort Sumter and Lincoln's subsequent call for volunteers caused many southern-born officers to resign their commissions. Terrill, however, left no doubt about where his loyalties lay, telegraphing the War Department and General Winfield Scott from Poughkeepsie, New York, on April 29, 1861 that "I am now and ever will be true to my oath and my country. No one has any authority to tender my resignation. I will be in Washington as soon as possible." Terrill was commissioned a captain in the 5th Regiment of Artillery in August 1861. He was one of sixteen Virginia-born officers in the regular army to tender his services to the Union. His younger brother James B. Terrill was commissioned a major of Virginia Infantry and served in the Confederate States Army. Their father served the Confederate States as the provost marshal of Bath County, Virginia. The elder Terrill wrote his son a scathing letter of disapproval saying "Can you be so recreant and unnatural as to aid in the mad attempt to impose tyranny upon your kith and kin? Do so and your name shall be stricken from the family records."