With the rank of Oberst (colonel), Rüchel fought against the First French Republic during the War of the First Coalition. During the Siege of Mainz from 10 April to 23 July 1793, he commanded a brigade consisting of the "Thadden" and "Legat" Fusilier Battalions and the "Wolframsdorf" Grenadier Battalion. After promotion to General-major, he led the Prussian contingent during the Second Battle of Wissembourg in December 1793. The units coming under his orders were eight battalions in the Infantry Regiments "Kleist" Nr. 12, "Wolframsdorf" Nr. 37, and "Hertzberg" Nr. 47, two foot jäger companies, three squadrons of the "Wolfradt" Hussar Regiment Nr. 6, one and one-half horse artillery batteries, and a half 6-pound foot battery.