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English summary: This book, which is the fruit of long years of research and reflection, presents through a succession of short chapters a sound historic overview of Julian's amazing career as a man of letters and emperor.Most of the work spans the brief period of his life when Julian, then emperor, strove to restore Hellenism, reformed paganism by means of a theocracy, and fought against Christianity. J. Bidez's contribution consists of demonstrating that the fight against Christianity was not the outcome of a plan made in advance and carried out in stages, but the result of a series of diverse events and reversals which occurred as the era unfolded. French description: Fruit de longues annees de recherche et de reflexion, cette synthese historique solide retrace l'etonnante carriere de Julien, homme de lettres et empereur, par une succession de chapitres courts. Le gros de l'ouvrage porte sur la breve periode de la vie de Julien ou ce dernier, devenu empereur, s'est applique a restaurer l'hellenisme, a reformer le paganisme par une theocratie et a lutter contre le christianisme. L'apport de J. Bidez est de montrer que le combat contre le christianisme n'est pas le fruit d'un plan etabli d'avance et realise par etapes mais la resultante d'une succession de mouvements et de revirements divers au gre des circonstances de l'epoque.