Back in Europe, responding to a characteristically devious provocation from Chancellor Bismarck, the French parliament voted to declare war against Prussia on 19 July 1870. Lambert's unit found itself close to Sedan where the French commander Marshal de MacMahon had decided to concentrate military resources in order to block an anticipated Prussian military advance through Belgium. By this time Lambert was serving with the 2nd Marine Infantry Regiment ("2e Régiment d'Infanterie de Marine") which formed part of the so-called Blue Division which in a series of savage engagements during the wider Battle of Sedan captured and then recaptured Bazeilles from I Royal Bavarian Corps under the command of General von der Thann.