Sir Michael Howard has observed that Douglas Haig was a military manager in the mould of Ulysses S. Grant and Dwight D. Eisenhower rather than one of the “Great Captains” of military legend. Unfortunately, Howard did not elaborate. To date, this crucial aspect of Haig’s role on the Western Front has not been explored. The contention of this thesis is that Haig was an exceptional military manager who pursued the organising principle of unity-of-effort within the BEF on the Western Front to facilitate the defeat of the German Army in concert with the Allies. In 1909, Haig established unity-of-effort as the first principle of war organization in FSR-II.1 Haig did not define the precept possibly in the belief that it was a commonplace. However, a study to establish the contemporary understanding has revealed that unity-of-effort was, and is, the raison d’être of all forms of human organization including the military. It was regarded as a tangible and effective principle and not a mere rhetorical gesture or oratorical flourish. Its nature was immutable, and uniquely coordinative. Unity-of-effort found expression in its compound character, which had distinct mental, physical and moral components, specific to each organization. The principle was considered to be a normative ideal, and not an absolute standard. Haig strove to optimise unity-of-effort by developing operational, organizational and administrative doctrine in pursuit of unity-of-mental-effort; by inculcating the teachings of doctrine through progressive training methods to achieve unity-of-physical-effort; and by promoting the will to fight through sustained morale and discipline to attain unity-of-moral-effort. Haig managed the process to attain unity-of-effort through the coordinative function of the General Staff.