When Napoleon invaded Russia, Osten-Sacken returned to military service at the head of a reserve corps, based in Volynia. He was given the task of defending the southern borders of the Empire against the possible invasion by Saxon and Austrian armies. In the battle near Volkovysk he defeated a French corps under General Jean Reynier. Following Reynier's defeat, Osten-Sacken crossed the border and invaded the Duchy of Warsaw and joining his forces with Count Mikhail Miloradovich, took Warsaw. Later he successfully operated against Prince Józef Poniatowski. His brilliant conquest of Poland won him the Order of Alexander Nevsky.