The Army of Mainz

Mainz

Mainz under the Holy Roman Empire, and previously was a Roman fort city which commanded the west bank of the Rhine and formed part of the northernmost frontier of the Roman Empire...

(Armée de Mayence) was a French Revolutionary Army

French Revolutionary Army

The French Revolutionary Army is the term used to refer to the military of France during the period between the fall of the ancien regime under Louis XVI in 1792 and the formation of the First French Empire under Napoleon Bonaparte in 1804. These armies were characterised by their revolutionary...

 set up on 9 December 1797 by splitting the Armée d'Allemagne

Armée d'Allemagne (1797)

The Army of Germany was one of the French Revolutionary armies, formed by a decree of the French Directory dated 29 September 1797 by merging the armée de Sambre-et-Meuse and the armée de Rhin-et-Moselle and commanded from the decree until 6 October by général Saint-Cyr under général Hoche...

 into the Armée de Mayence and the Armée du Rhin

Armée du Rhin

The Army of the Rhine is the overall name for one of the main French Revolutionary armies, that operated in the German theater along the River Rhine...

. Part of it split off on 4 February 1799 to form the armée d'Observation, though part of the armée d'Observation then re-merged into the Armée de Mayence on 28 March that year (with the remainder forming the Armée du Danube).