At the beginning of the First World War, Henri Mathias Berthelot was recognized as one of France's most brilliant younger generals. His sharp intelligence, prodigious organizational talents, and verbal skills had made him the trusted assistant to a succession of French chiefs of staff including Brugere, Lacroix, and finally Joffre. As the latter's 'right arm' in implementing Plan XVII in 1914, he shared responsibility for the earliest defeats of the French Army, but also for the remarkable recovery that followed. His career as a field commander began with a reverse at Soissons (January 1915) but acclaimed successes followed, including the Labyrinth (Artois 1915) and Mort Homme (Verdun 1916) and also the second Battle of the Marne in which he commanded the French V Army (July-September 1918). Following the war, he contributed to the reintegration of Alsace and Lorraine into France in his role as military governor of Metz (1919-1921) and Strasbourg (1923-1926). Simultaneously, he played an influential role on the Conseil Superieure de Guerre in its debates over the defense of France which eventually produced the Maginot Line. While, unfortunately, Berthelot's military career in France has not received the attention it deserves, his service in Romania as head of two French military missions (October 1916-March 1918) and (October 1918-May 1919) made him a national hero in that country. Arriving in Bucharest in the midst of a demoralizing defeat at the hands of the Central Powers in 1916, Berthelot's indomitable optimism and will to resist energized the Romanian political and military leadership. With the assistance of Berthelot, the Romanian army was rebuilt, a new Austro-German assault brilliantly repulsed in 1917, and the consequences of the Russian Revolution for Romania blunted. At the end of the war, when Allied leaders vacillated over allowing the Romanians to occupy the territory promised by the Allies in the Treaty of Alliance of 1916, Berthelot encouraged them to act on their own. His advocacy of Romania's revindications and his forceful promotion of her as the major instrument of French policy in Southeastern Europe alienated his superiors and eventually led to his recall. Meanwhile, the course of events, including the emergence of Bolshevik threats in South Russia and Hungary, led Paris to adopt his recommendations. Berthelot's strong, yet sensitive tutelage of Romania 1916-1919 and his warm, public identification with her people and their aspirations endeared him to the elite and masses alike. In return, they enshrined him in the first rank of their pantheon of the founders of 'Greater Romania'. This study is based on the private, unpublished 'Sourvenirs', papers, and correspondence of the general. In addition, it utilizes the military and diplomatic archives in France, Romania, Italy, Russia, Austria, Germany, England, and the United States.