With the defeat of Marshal Bazaine's Army of the Rhine at Gravelotte, the French were forced to retire to Metz, where they were besieged by over 150,000 Prussian troops of the First and Second Armies. Napoleon III and MacMahon formed the new French Army of Châlons, to march on to Metz to rescue Bazaine. Napoleon III personally led the army with Marshal MacMahon in attendance. The Army of Châlons marched northeast towards the Belgian border to avoid the Prussians before striking south to link up with Bazaine. The Prussians, under the command of Field Marshal Count Helmuth von Moltke, took advantage of this maneuver to catch the French in a pincer grip. He left the Prussian First and Second Armies besieging Metz, except three corps detached to form the Army of the Meuse under the Crown Prince of Saxony. With this army and the Prussian Third Army, Moltke marched northward and caught up with the French at Beaumont on 30 August. After a sharp fight in which they lost 5,000 men and 40 cannons, the French withdrew toward Sedan. Having reformed in the town, the Army of Châlons was immediately isolated by the converging Prussian armies. Napoleon III ordered the army to break out of the encirclement immediately. With MacMahon wounded on the previous day, General Auguste Ducrot took command of the French troops in the field.

What factor forced the French to retreat to Metz?