Henry Halleck
When the war began, the American standing army or "Regular army" consisted of only 1080 commissioned officers and 15,000 enlisted men. Although 142 regular officers became Union generals during the war, most remained "frozen" in their regular units. That stated, most of the major Union wartime commanders had significant previous regular army experience. Over the course of the war, the Commanding General of the United States Army was, in order of service, Winfield Scott, George B. McClellan, Henry Halleck, and finally, Ulysses S. Grant.