When one thinks about the impact of Mahan on American military thought and practice, one naturally thinks of Alfred Thayer Mahan, the naval historian and theorist of the late nineteenth and early twentieth century. But there was another notable Mahan – Dennis Hart Mahan (USMA, 1824), Alfred’s father, who taught for many years at West Point in the mid-nineteenth century and whose personality and views influenced Civil War commanders on both sides of the divide. The elder Mahan plays a minor role in Robert L. O’Connell’s new popular biography, Fierce Patriot: The Tangled Lives of William Tecumseh Sherman. (The book focuses on Sherman’s character and his larger role in American nineteenth century history, not just on the Civil War or his peculiar military excellence.)
Mahan drew from one of the two great interpreters of Napoleon, Antoine Henri, Baron de Jomini (the other being Clausewitz, now very popular among the U.S. military, but not translated into English until after the Civil War). The U.S. Military Academy during this period was focused on engineering and the nuts and bolts of things military (and often on just nuts and bolts). Mahan’s senior seminar, in which he discussed military theory, was something of the exception to the rule. One of this students, Henry W. Halleck (who later served as Lincoln’s principal military aide), became another disciple of Jomini. He authored Elements of Military Arts and Sciences (1846) and later translated Jomini’s Life of Napoleon. The military historian Russell F. Weigley characterized Mahan and Halleck as the founders of American strategic studies.
Jonimi’s principles of war were as immutable as the geometry he sought to emulate: bring to bear the maximum possible force at the decisive point in the theater of operations, such that the enemy can muster only an inferior part of his strength there. This depended on the proper ordering of one’s own line of communications in relation to the enemy – which almost inevitably meant operating on interior lines. Jomini’s logic pointed less toward the destruction of the enemy’s armed forces and more to the control of key geographic locations.
Mahan was not merely an epigone of Jomini, but rather applied his principles to American circumstances and to Mahan’s own sense of the conduct of war. According to Allen C. Guelzo in Fateful Lightning: A New History of the Civil War, Mahan argued that the Napoleonic search for the decisive battle, in the form of the offensive, must be qualified by a realistic appreciation for the risks the offensive might run.
Like Jomini, Mahan encouraged future generals to maneuver— but not, like Jomini, in order to gain advantage for an attack. Instead, fully aware that American armies were bound to the use of militia and volunteers, the principal object in Mahan’s teaching was to seize and occupy enemy territory, and eventually force the enemy to launch an attack on one’s own defensive fortifications. That required intensive training in the construction of major fortifications and instruction in the creation of temporary fieldworks on the battlefield, and that was what Mahan and West Point offered. . . . Mahan took an academy that had been designed mostly for the defensive protection of American territory through the construction and garrison of fortification, combined it with a military tradition shaped by political mandates from Congress to favor a defensive mission, and raised the art of defense to an American science. . . . The American regular army officer in 1861 was thus presented with a series of contradictions: tactics books that encouraged officers to take the offensive and make the enemy’s army their objective, and a professional military culture that looked to occupy enemy territory and fight a defensive war from behind fortifications.
Mahan’s pupils applied his teachings their own distinctive, and far from uniform, way. It is an interesting case study in how strategic authority is transmitted and adapted. George McClellan was widely regarded as Mahan’s best student, and one can see Mahan’s influence in McClellan’s concept of war (an active modification of Winfield Scott’s original Anaconda Plan), one which did not rely on decisive battles. For his part, Ulysses S. Grant claimed that had never read Jomini. Rather, he operated according to a basic rule: “The art of war is simple enough. Find out where your enemy is. Get at him as soon as you can. Strike him as hard as you can, and keep moving on.”
Sherman remembered Mahan affectionately and took great pride in a letter from Mahan, offering congratulations on the Vicksburg campaign. In his memoirs, Sherman makes only one substantive reference to his old teacher: “It was one of Prof. Mahan’s maxims that the spade was as useful in war as the musket, and to this I will add the axe,” referring to the use of the axe to build field works. (I reprint below the section from Sherman’s memoirs that makes this reference; they were published after the Franco-Prussian war, and this passage involves Sherman’s reflections on contemporary character of war in general.)
The geographic relationship between the Union and the Confederacy raised the question of interior and exterior lines of communication, both at the operational and strategic level. Halleck in particular was obsessed with applying Jomini’s principles to the conduct of the war, including in the planning of Grant’s overland campaign in 1864. Grant and Sherman, for their part, famously developed innovative ways to overcome the problem in the West, including the control of key rivers and cutting loose from lines of supply and systematically foraging off the land.
Civil war experts could cite many more such influences, especially on the Southern side. To note a few:
Michael Phipps argues here for the importance of Mahan’s influence on the conduct of the battle of Gettysburg. See also Edward Hagerman, “From Jomini to Dennis Hart Mahan: The Evolution of Trench Warfare and the American Civil War,” Civil War History, 12 (1967), 197–220.
There are also the Memoirs of General William T. Sherman. By Himself (1875)
Modern wars have not materially changed the relative values or proportions of the several arms of service: infantry, artillery, cavalry, and engineers. If any thing, the infantry has been increased in value. The danger of cavalry attempting to charge infantry armed with breech-loading rifles was fully illustrated at Sedan, and with us very frequently. So improbable has such a thing become that we have omitted the infantry-square from our recent tactics. Still, cavalry against cavalry, and as auxiliary to infantry, will always be valuable, while all great wars will, as heretofore, depend chiefly on the infantry. Artillery is more valuable with new and inexperienced troops than with veterans. In the early stages of the war the field-guns often bore the proportion of six to a thousand men; but toward the close of the war one gun; or at most two, to a thousand men, was deemed enough. Sieges; such as characterized the wars of the last century, are too slow for this period of the world, and the Prussians recently almost ignored them altogether, penetrated France between the forts, and left a superior force “in observation,” to watch the garrison and accept its surrender when the greater events of the war ahead made further resistance useless; but earth-forts, and especially field-works, will hereafter play an important part in war, because they enable a minor force to hold a superior one in check for a time, and time is a most valuable element in all wars.
It was one of Prof. Mahan’s maxims that the spade was as useful in war as the musket, and to this I will add the axe. The habit of intrenching certainly does have the effect of making new troops timid. When a line of battle is once covered by a good parapet, made by the engineers or by the labor of the men themselves, it does require an effort to make them leave it in the face of danger; but when the enemy is intrenched, it becomes absolutely necessary to permit each brigade and division of the troops immediately opposed to throw up a corresponding trench for their own protection in case of a sudden sally. We invariably did this in all our recent campaigns, and it had no ill effect, though sometimes our troops were a little too slow in leaving their well-covered lines to assail the enemy in position or on retreat. Even our skirmishers were in the habit of rolling logs together, or of making a lunette of rails, with dirt in front, to cover their bodies; and, though it revealed their position, I cannot say that it worked a bad effect; so that, as a rule, it may safely be left to the men themselves: On the “defensive,” there is no doubt of the propriety of fortifying; but in the assailing army the general must watch closely to see that his men do not neglect an opportunity to drop his precautionary defenses, and act promptly on the “offensive” at every chance.