De Négrier was attached to the Imperial Japanese Army as an official French observer during the Russo-Japanese War, and published an account of the war, Lessons from the Russo-Japanese War (London, 1905), immediately after its close.
De Négrier dilekatkan pada Angkatan Darat Kekaisaran Jepang sebagai pengamat resmi Prancis selama Perang Rusia-Jepang, dan menerbitkan laporan perang, Lessons from the Russo-Japanese War (London, 1905), segera setelah penutupan.