Ferdinand's younger brother Léon de Montesquiou was involved in the political and intellectual debates of the Third Republic and the "Belle Époque". Originally a left-leaning liberal, he joined the hard-right "Ligue de la patrie française" in 1898. He became a founding member of the larger "Action Française" the following year, earning his reputation as both a nationalist-antisemitic doctrinaire and a writer on Romanian subjects. Having lost his officer's rank for his anti-Dreyfusard propaganda, he enlisted as a regular soldier on the Western Front. Léon was eventually reinstated as a Lieutenant, but was killed in action during the Second Battle of Champagne, at Souain. Another brother, Raoul, who lived to 1934, spent most of his time at the Château de Courtanvaux in Bessé-sur-Braye.