Having voted war credits on 15 July, France formally declared war on Prussia four days later. The declaration detonated explosions of patriotic feeling in Paris. The mass demonstrations before Emile Ollivier's hôtel on the Place Vendôme, General Jean-Baptiste Montaudon nervously watched. Having declared war in the Corps Législatif, Ollivier was the man of the hour; he relished the attention, periodically appear on his balcony to wave to the surging crowds. In Paris, troops were mobbed as they marched to the eastern station. Civilian bystanders pressed in on them from all sides and joined their ranks, shouldering packs and rifles, standing drinks at sidewalk cafés, and bellowing “à Berlin! À bas les prussiens!” Citizens thronged the Tuileries Palace day and night waiting for their Napoleon to ride out and take command of the army. The French provinces were at least as excited. Troops rolling to the front in railcars recalled that even remote train stations were crowded with spectators yelling encouragement, shoving flasks of wine through the windows, and enjoining the men to rout the “German blockheads” – “têtes carrés allemands.”

French expectations ran high, and for Louis-Napoleon's troops everything turned on the speed with which they could deploy to the frontiers. Although the French would eventually be outnumbered by the Prussians, they would have several weeks in which to strike with their long-service regulars before Prussia could collect its reserves.