A Delaware native and West Pointer (1842), Sykes was a veteran of infantry service in both the Seminole and Mexican wars, earning a brevet in the latter. At 1st Bull Run he commanded the only regular army infantry on the field, an eight-company battalion from various regiments, and was highly effective in slowing the rout of the volunteers. He then commanded the regulars near Washington and in the midst of the Peninsula Campaign was given charge of a division composed mostly of regular army units. He had already fought at Yorktown and in divisional command participated in the Seven Days fighting. He was at 2nd Bull Run and in reserve at Antietam. Given a second star in the volunteer service, he fought at Fredericksburg and Chancellorsville before taking charge of the 5th Corps upon George G. Meade's assumption of army command just prior to Gettysburg. There he fought in support of the hard-pressed 3rd Corps on the second day. Mustered out of the volunteer service on January 15, 1866, he reverted to his regular army rank and died on active duty in Texas as colonel, 20th Infantry, and brevet major general for the war.