It is an interesting study to examine into the causes or motives which have produced military books of the higher order; for we are thus vouchsafed an insight into the writer's genius, and an intelligence of the circumstances amidst which he wrote, and of which he was often an important controller. The Archduke Charles wrote his "Grundsätze der Strategie," etc., as a vindication of his splendid movements in 1796, against the French armies of the Rhine and the Sambre-et-Meuse; and it has remained at once a monument to his achievements and a standard text-book in military science. Marmont, the Marshal Duke of Ragusa, collecting the principles of the art of war from "long and frequent conversations with Napoleon, twenty campaigns, and more than half a century of experience," has given us, in his "Esprit des Institutions Militaires," a condensed view of his own military life, as complete, if not as pleasantly diffuse, as his large volumes of "Mémoires." Jomini, from an extended experience, and a study of the genius of Napoleon, which his Russian position could never induce him to undervalue, has produced those standard works which must always remain the treasure-houses of military knowledge. We admire veracity, but let no soldier confess that he has not read the "Vie Politique et Militaire," and the "Précis de l'Art de la Guerre." But, in all these cases, the _litera scripta_ has been but the closing act,--the signing of the name to History's bead-roll of passing greatness,--the _testamentum_ of the old soldier whose _personalty_ is worth bequeathing to the world. The work before us, although of great value and present importance, is of a very different character; as a glance at the circumstances which produced it will show. It has, however, we would fondly hope, anticipated for its youthful author a greater success. In 1855, Mr. Jefferson Davis, then Secretary of War, sent a military commission to Europe, composed of Major Delafield of the Engineers, Major Mordecai of the Ordnance, and Captain McClellan, just promoted from a Lieutenancy of Engineers to a Captaincy in the Cavalry. Major Delafield was charged with the special subject of Engineering; Major Mordecai with Ordnance and Gunnery; and to Captain McClellan was assigned the duty of a general report upon the Organization of Armies, with a special hearing upon the formation of Infantry and Cavalry. Each of these gentlemen has written a book, and that of McClellan, originally published as a Report to the Secretary of War,--in unmanageable quarto, and at a more unmanageable price,--is now issued, in the volume before us, with the very appropriate title, "The Armies of Europe," and in a convenient form for the eye and the purse.