The study of the private archives of the Château de Lagrange (Moselle, France) allows for the writing of the biography of Jean de Bertier and gives him his proper place within the framework of the evolution of a dynasty of nobility over about a century (around 1840-1940). What emerges from this initiative is the primacy of economic matters. Jean de Bertier displays a genuine faculty to adapt to both circumstantial and structural evolutions through the diversification of the family wealth. When faced with inflation, he starts focusing on industrial stocks, including iron and steel, by increasing his investment in the ARBED. So as to cope with taxation, he takes advantage of the opportunities offered by borders in order to develop a transnational estate in which Luxembourg is of paramount importance. Notwithstanding a consistent loss of the family fortune he is not able to prevent given the historical context, his efforts still allow him to preserve the family's rank within the hierarchy of assets. The Bertier family thus keeps on with an aristocratic way of life that's typical of high society. Jean de Bertier's military enrolment also fits the perpetuation of a nobiliary value system promoting duty and patriotism. Analysing his military career confirms the central importance of legalism within the army during the Belle Époque. It reveals the growing number, during World War I, of liaison, instruction and intelligence operations that often remain little-known. The Great War and its consequences represent a major turning point in Jean de Bertier's life. In 1919, he resigns from the army to embrace a political career in the peculiar remit of an Alsace-Lorraine that is French again, more particularly in the Moselle department. Tracking down Jean de Bertier's journey helps demonstrate the primordial importance, for any elected official, of efforts of intercession and recommendation, as well as the vital role of the defence of regional interests. Once more, he proves his ability to adapt as he successfully becomes part of booming networks (the Lorraine Republican Union, the press) and tempers political views that remain quite conservative with moderate practices. His election to the French senate in 1922 is the crowning achievement of his political career. It doesn't give him, however, the stature of Robert Schuman, his most famous colleague. The two men are false rivals who evolve within the same transnational space, focused on Moselle and Luxembourg, without sharing the same ideas nor practices.