An Army is a collection of armed men obliged to obey one man. Every change in the rules which impairs the principle weakens the army.

Sherman is defining an army less as a patriotic institution than as a mechanism of command: armed force becomes effective only when individual wills are subordinated to a single directing authority. The second sentence argues that discipline is not merely one virtue among others but the organizing principle that makes an army function at all; reforms that dilute clear obedience—however well-intentioned—risk turning coordinated force into a loose aggregation of fighters. Read in light of Sherman’s hard-earned Civil War experience, the remark reflects a professional soldier’s belief that unity of command and strict adherence to rules are prerequisites for operational effectiveness, especially under the stress, fear, and friction of combat.