Its loss was not redeemed by the French offensive in Alsace and Lorraine. On 7 August a weak French force advanced through the Belfort gap and, finding still weaker forces to oppose it, proceeded to occupy Altkirch and Mulhouse, while a proclamation by General Joffre announced the approaching liberation of the provinces torn from France in 1870. It was a feeble and ill-conceived effort to snatch a political advantage out of a forbidding military situation. German reinforcements swept up from Colmar and Neu Breisach, and on the both the French were back within a few miles of the frontier, leaving their sympathizers to the vengeance of their enemies. More legitimate though not more successful was the French thrust in Lorraine. It had other motives than the political: it would, if pushed home, menace the left of the German armies in Belgium and disturb their communications; and a smaller success would avert the danger of a German advance in Lorraine which would threaten the right of the French on the Meuse. Accordingly, Generals Pau and de Castelnau, commanding the armies of Alsace and Lorraine respectively, ordered a general advance on the 10th. At first it met with success: the chief passes of the Vosges from Mt. Donon on the north to the Belfort gap were seized; counter-thrusts by the Germans towards Spincourt and Blamont in the plain of Lorraine were parried; Thann was captured, Mulhouse was re-occupied, and the Germans looked like losing Alsace as far north as Colmar. German Lorraine seemed equally insecure, for on the 18th Castelnau's troops were in Saarburg cutting the rail and roads between Strassburg and Metz. The Germans, however, were not unprepared: their Fifth Army, under the Crown Prince Rupprecht of Bavaria, came down from Metz and fell upon the exposed French left, which was routed with great losses in guns and prisoners on the 21st. Not only did the invasion collapse, but the Bavarians pushed across the French frontier nearly as far as Toul and occupied Lunéville, compelling also a French retreat from the passes of the Vosges. General Pau had soon to follow suit and retire again from Mulhouse and all but the south-west corner of Alsace.
The operations in Alsace and Lorraine had dismally failed to discount the advance of the Germans through Belgium or even to impede the march of their centre through Luxemburg and the Ardennes. At the end of three weeks France was still in the throes of mobilization: the original scheme of defence along the Franco-German frontier had been upset by the German attack through Belgium; and second thoughts had fared little better at Namur. The shortest line of defence after the Germans had broken through at Liège was one running from Antwerp to Namur, and the shortest line is imperative for the weaker combatant. But the Germans were well across it when they entered Brussels, and with the fall of Namur the hinge upon which depended the defence of the northern frontier of France was broken. It was to an almost forlorn hope that the British Army was committed when it took its place on the left of the French northern armies at Mons to encounter for the first time since Waterloo the shock of a first-rate European force. But for its valour and the distraction caused by the Russian invasion of East Prussia, Paris and possibly the French armies might not have been saved.