The battle of Pirmasens (14 September 1793) was a costly defeat for the French on the west bank of the Rhine in the aftermath of the fall of Mainz. While the Prussians and Austrians were besieging Mainz, the French armies of the Moselle and of the Rhine remained in a defensive position between Saarbrucken and the Rhine, with the Army of the Moselle on the left and the Army of the Rhine in the Lines of Wissembourg. A half-hearted relief effort in May failed, and on 23 July Mainz surrendered.
The Austrians and Prussians took nearly two months to move south towards the French lines. The Austrians advanced towards the French right (east of the northern end of the Vosges), while the Prussians advanced rather half-heartedly to Kaiserslautern, and then on to Pirmasens, from where they commanded one of the passes across the Vosges.
In early September the French and the Austrians clashed around Bodenthal, a key position to the west of Wissembourg. General Würmser's Austrians took the position off the French, a move that threatened the best line of communication between the two French armies. On 12 September the French launched a counterattack, but this was repulsed.
The next French attack, on 14 September, involved both armies. Part of the Army of the Rhine, under the command of General Ferrette, captured Bodenthal, but the other flank of the attack was less successful.
12,000 men from the Corps of the Vosges were ordered to advance east from their camp at Hornbach to attack the Prussian position at Pirmasens. The Prussian commander-in-chief, the Duke of Brunswick, was based at Pirmasens with a strong force in a good defensive position. The Prussians detected the French well before they reached their target. While the French generals wanted to turn back to Hornburg, the representatives of the government insisted on an attack. The French formed up into three columns, each of which attacked along one of three valleys that climb up to the town.
The French columns advanced some way up these valleys before they came under a heavy flanking fire from the heights between them. The column on the right was forced to retreat first, soon followed by the left and centre. In the battle and the retreat that followed the French lost 4,000 men and twenty-two guns, one third of the original attacking force.
The French defeat at Pirmasens may have played a part in encouraging the Allies to make their attack on the Lines of Wissenbourg (13 October 1793), a success that for a short period appeared to expose Alsace and Lorraine to invasion.