In 1812 he went on diplomatic mission to Prussia; this mission had the purpose of ascertaining King Frederick William III towards his attitude in case France went to war with Russia. In returning to Paris, de Narbonne-Lara advised Napoleon not to invade Russia, but his advice was not followed; and so he reassumed his place in the Imperial Army on the Russian border. When he arrived there, he was sent by Napoleon to the city of Wilna, where the Tsar was; the comte de Narbonne-Lara was the bearer of an ultimatum, to which Alexander I categorically refused.