1804 Duke Charles Augustus wuxuu galay siyaasada Yurub isagoo guursaday wiilkiisa iyo dhaxalka Charles Frederick Grand Duchess Maria Pavlovna, walaasheed Emperor Alexander I ee Ruushka. Si kastaba ha ahaatee, wuxuu isku xidhay Prussia dagaalkii Isbahaysiga Afaraad
kadib guuldaradii ka soo gaartay dagaalkii Jena-Auerstedt waxaa lagu qasbay inuu ku biiro isbahaysiga Napoleon ee Rhine sanadkii 1806. Sanadkii 1809 Saxe-Weimar iyo Saxe-Eisenach, oo ay ku midoobeen oo keliya qofka duke, ayaa si rasmi ah loogu daray Duchy of Saxe-Weimar-Eisenach.