Antoine-Henri Jomini was the most celebrated writer on the Napoleonic art of war. Jomini was present at most of the most important battles of the Napoleonic Wars. His writing, therefore, is the most authoritative on the subject.
"The art of war, as generally considered, consists of five purely military branches,-viz.: Strategy, Grand Tactics, Logistics, Engineering, and Tactics. A sixth and essential branch, hitherto unrecognized, might be termed Diplomacy in its relation to War. Although this branch is more naturally and intimately connected with the profession of a statesman than with that of a soldier, it cannot be denied that, if it be useless to a subordinate general, it is indispensable to every general commanding an army." -Antoine-Henri Jomini