At the start of the American Civil War, the U.S. Army had five mounted regiments. Cooke commanded the 2nd Dragoons, which was redesignated the 2nd U.S. Cavalry. As they prepared to ride into their first battles, they had the potential opportunity to learn from the two-volume manual on cavalry tactics written by Cooke in 1858, but not published until 1862. It was a controversial work at the time and the War Department chose not to make it the basis for official doctrine. Cooke espoused the value of mounted attacks as the primary purpose for cavalry forces; others, more sensibly, realized that the emergence of the rifled musket as an infantry weapon made the classic cavalry charge essentially obsolete and recommended a mission emphasis on reconnaissance and screening. Even those who agreed that cavalry charges retained some value found reasons to disagree with Cooke. A prominent theory of cavalry charges at the time, endorsed by future generals Henry W. Halleck and George B. McClellan, was that the cavalry should be deployed in double ranks (a regiment would deploy in two lines of five companies each), which would increase the shock effect of the charge by providing an immediate follow-up attack. Cooke's manual called for a single-rank formation in which a battalion of four companies would form a single line and two squadrons of two companies each would cover the flanks. A third battalion would be placed in reserve a few hundred yards to the rear. Cook believed that the double-rank offensive promoted disorder of the horses in the ranks and would be difficult to control.