The siege of Mainz of 14 December 1794-29 October 1795 was an unsuccessful French attempt to recapture the key Rhineland city which they had briefly held between October 1792, when it had fallen after a three day siege, and 23 July 1793, when the starving defenders had surrendered to the Prussians.
This third siege began as a blockade late in 1794, under the command of General Kléber, who had recently captured Maastricht (17 September-14 November 1794). Kléber was aware that he had little chance of capturing Mainz – his army was not large enough to conduct a regular siege, he lacked the heavy artillery needed to destroy the walls, and two Austrian armies under Clairfayt and Würmser were formed on the far side of the Rhine. For most of its ten month duration, this siege was thus simply a blockade on the western bank of the Rhine.
The siege was given more impetus in the autumn of 1795, when the French launched a new offensive across the Rhine. South of Mainz the Army of the Rhine and the Moselle, under General Pichegru, captured Mannheim, while to the north the Army of the Sambre and the Meuse, under General Jourdan, crossed the river in early September and advanced towards Nassau. For a moment the French had a chance to inflict a serious defeat on the Austrians, but the moment passed. Pichegru opposed a combined operation. The two commanders agreed that while they waited for instructions from Paris Jourdan should move to besiege Mainz, while Pichegru continued to operate from Mannheim.
This delay gave the Austrians time to restore the situation. Würmser concentrated against Mannheim, freeing Clairfayt to deal with Jourdan. Jourdan's army was spread out from Mainz, at the right, through Hoechst, towards Nassau, and was theoretically protected by neutral territory at Frankfort, but Clairfayt marched through some of this territory, and by 11 October was in a position to march around the French left flank. At a council of war Jourdan's officers opposed any idea of attacking the Austrians, and voted for a retreat. On the night of 16 October the army began to pull away to the north, eventually crossing the Rhine between Neuwied and Dusseldorf by 20 October.
This move left the French troops blockading Mainz dangerously isolated. Jourdan was now some way distant to the north west, close to Coblenz, while Pichegru was at Mannheim. General Clairfayt decided to break the blockade of Mainz. With the east bank of the Rhine in Austrian hands, he was able to increase the strength of the garrison in Mainz, and on 29 October the Austrians launched a surprise attack from within the city. The French were forced out of their siege lines, and lost all of their siege train. Clairfayt then turned south to attack Pichegru. The campaign came to an end after the Austrians recaptured Mannerheim, and proposed an armistice, which lasted until the spring of 1796. Mainz was surrendered to France in the treaty of Campo Formio of 17 October 1797, and again in the peace of Luneville in 1801, and remained in French hands until 1814.