The Battle of Lorraine, 14 August-7 September 1914 (First World War), began as part of the then current French war plan (Plan XVII). This called for a general offensive across the Franco-German border at the outbreak of war.
There were a variety of good reasons for launching this offensive. First, Alsace-Lorraine had been French territory until 1871, when it was seized by the new German Empire after the Franco-Prussian War. Recapturing Alsace-Lorraine was thus a major French preoccupation. Second, it was believed that the Russians would need more time to mobilise their vast armies than the French or Germans. A French offensive would relieve pressure on the Russians, and win time for the Russian steamroller to win the war. Third, the French were aware that as time went on, they would be increasingly outnumbered by the Germans. An immediate offensive was the best chance of taking advantage of the large French peacetime army.
The French allocated two armies to the offensive in Lorraine – the First Army, under General Auguste Dubail to the south and the Second Army under General Édouard de Castelnau to the north. Between them these armies contained six Corps. To their right was the small Army of Alsace, which had already made one unsuccessful attack into Alsace (Battle of Mulhouse).
They were opposed by two German armies. Dubail faced the German Seventh Army, under General Josias von Heeringen, while Castelnau faced the German Sixth Army under Crown Prince Rupprecht of Bavaria. These armies contained eight Corps. The French would be outnumbered.
The Germans were following a modified version of the famous Schleiffen Plan. The main German effort was to come further north, and would involve an invasion of neutral Belgium and a great wheeling movement through north east France. If all went well part of the German army would pass west of Paris, but even if that was not achieved, it was hoped that the great wheeling movement would envelop the main French armies, trapping them against the German border and forcing their surrender. As part of this plan, the German armies in Alsace-Lorraine were to retreat east, allowing the French to advance into Alsace-Lorraine. Every mile the French armies moved east would make it harder for them to intervene in the crucial battle happening to the North West.
The French attack began on 14 August. For four days the French advanced without meeting serious German opposition. The two German armies, under the overall command of General Krafft von Delmensigen, maintained contact with the French, but withdrew from any serious confrontation. On 18 August the French VIII Corps captured Sarrebourg.
During the advance a gap had opened up between the two French armies. On the night of 19-20 August, Dubail launched an attack designed to close this gap. The French attack ran head on into a full German counterattack. This was the second part of the German trap, and was intended to pin the French armies in place. At this point the Battle of Lorraine became part of the Battle of the Frontiers of France.
The German counterattack overwhelmed the Second Army. After the fighting on 20 August, the majority of the army was forced to pull back to the River Meurthe, its starting point six days earlier. Only the XX Corps, under General Ferdinand Foch, held its ground (Battle of Morhange). The retreat of the Second Army forced the First Army to pull back to the same line.
In the original German plan, that would have marked the end of the battle. The French would have been pinned in place on the Meurthe, unable to move many troops to the crucial northern front. Instead, Prince Rupprecht and General Heeringen convinced von Moltke to let then launch a series of counter attacks on the French lines. These began on 25 August and continued through the first week of September. They did not have the expected results. The French lines held. The fighting on the Alsace-Lorraine front became increasingly static during this period. This allowed General Joffre to move some troops from this area back towards Paris and the Marne. Amongst the men moved west was Ferdinand Foch, promoted to command a new Ninth Army, which would play a significant role in the battle of the Marne.