In 1808, Emperor Alexander sent Chernyshov as his private messenger to Napoleon, who at that time started the Peninsular War in Spain. Chernyshyov reached him in Bayonne, taking notes of French military development on his journey. He managed to establish a personalized link between two emperors and earned Napoleon's trust. Next year, tsar Alexander assigned Chernyshyov to be his permanent private envoy to Napoleon (parallel to regular Ambassador of Russia). Chernyshyov, a member of Napoleon's retinue, accompanied French headquarters throughout the War of the Fifth Coalition, and received the Légion d'honneur for the Battle of Wagram. In October 1809, he was promoted to the rank of Rittmeister of the Guards, next year - to Colonel.