Mac-Mahon had intended to march his army, accompanied by Napoleon III, from Châlons-sur-Marne northeast toward Metz, to relieve the French Army of the Rhine, which was trapped at Metz. Moltke learned of Mac-Mahon's movements through newspaper reports and rapidly moved the newly formed Army of the Meuse, under Crown Prince Albert of Saxony, north to intercept Mac-Mahon. In three small engagements on the Meuse River (August 29 to 31), the Germans forced Mac-Mahon to fall back to the fortress at Sedan. While Mac-Mahon tried to consider whether to try again to smash through eastward toward Metz or instead to retreat west to Paris, Moltke moved up the 3rd Prussian Army, under Crown Prince Frederick William (Frederick III), to complete the encirclement of Sedan.

The wounding of Mac-Mahon early on September 1 caused extreme confusion in the French command, allowing the Germans to carry on their encirclement without serious opposition. Thereafter, the desperate French efforts to break out, including massive cavalry charges, led to nothing but high casualties. After the German artillery had pounded the French position in an all-day bombardment, the Germans launched their main attack in the afternoon. Emperor Napoleon III realized that the position was hopeless. He surrendered, and the next morning he and 83,000 French soldiers became prisoners of war. The French had lost 3,000 men killed, 14,000 wounded, and 21,000 missing or captured. German losses totaled 9,000 men killed and wounded. As the victorious Germans marched toward Paris, a popular uprising there on September 4 toppled the government of the Second Empire and set up a provisional republican government.