Zook's first combat was during the Seven Days Battles of 1862. His regiment was assigned to William H. French's brigade in Edwin V. Sumner's division of the Army of the Potomac, under Maj. Gen. George B. McClellan. Zook was personally scouting far out in front of his regiment in the run-up to the Battle of Gaines' Mill, got behind enemy lines, and found that Confederate Maj. Gen. John B. Magruder was conducting an elaborate deception, making it appear that he had significantly more troops in his sector than he actually had. Zook's discovery was reported up to McClellan, but it was ignored, and Union troops that could have been used successfully elsewhere remained tied down.