In the spring of1855, Jefferson Davis, then serving as secretary of war of the United States, sent three officers to Europe to learn about ‘the practical working of the changes which have been introduced of late years into the military systems of the principal nations. In particular, he asked these soldiers to observe the armies then fighting in Crimea; to visit military facilities in England, France, Prussia, Austria, and Russia; and ‘where they were of sufficient value to our service to warrant expenditure’, to obtain ‘new books, drawings, and patterns of arms and equipment’.
Each of the three members of the ‘military commission to Europe’ wrote reports that described the discoveries that they made within their peculiar fields of expertise. Thus, while Richard Delafield, of the Corps of Engineers, wrote extensively about fortress warfare and Alfred Mordecai, of the Ordnance Department, spilled much ink on the subject of firearms and artillery pieces, the (soon-to-be-famous) George B. McClellan devoted a great deal of attention to the organization and tactics of armies in the field.
The published versions of these reports can be found on the websites of both the Hathi Trust and the Internet Archive. (The links will take you to the best copy of each that I found on the latter service.)
Richard Delafield Report on the Art of War in Europe in 1854, 1855, and 1856 (Washington: George W. Bowman, 1869)
George B. McClellan Report of the Secretary of War Communicating the Report of Captain George B. McClellan (Washington: A.G.P. Nicholson, 1857)
George B. McClellan The Armies of Europe (Philadelphia: J.B. Lippincott, 1861)
Alfred Mordecai Report of the Military Commission to Europe in 1855 and 1856 (Washington: George W. Bowman, 1860)