After serving as a staff officer, he led a cavalry regiment with distinction during the Italian and Swiss expedition of 1799.

In 1804, he was selected to work closely with Archduke Charles, Duke of Teschen in implementing reforms in the Austrian army.

He became an aide-de-camp to Holy Roman Emperor Francis II in 1794 while serving in the Low Countries theatre of the War of the First Coalition.

During the Rhine Campaign of 1795, he was appointed Adjutant general to François Sébastien Charles Joseph de Croix, Count of Clerfayt and later to Dagobert Sigmund von Wurmser.

In a skirmish on 30 September, his troopers helped cover the retreat of Alexander Korsakov's Russian army after its defeat in the Second Battle of Zurich.

[5] Hohenlinden was a catastrophe in which the Austrian army lost 798 killed, 3,687 wounded, and 7,195 men and 50 guns captured.

Consequently, Emperor Francis II promoted Charles to Field Marshal and appointed him president of the Hofkriegsrat, the Imperial War Council, on 9 January 1801.

The British envoy Arthur Paget worried about Grünne's "pacifist disposition and admiration of Bonaparte".

Against the advice of Charles, but with the emperor's permission, Mack's army invaded Bavaria and advanced to Ulm.

[1] On 2 December 1805, Napoleon crushed the Austro-Russian army at the Battle of Austerlitz, ending the Third Coalition and defeating Austria in humiliating fashion.

[13] In the aftermath, Archduke Charles selected Anton Mayer von Heldensfeld as his chief of staff.

[1] Napoleon's invasion of Spain provoked the Dos de Mayo Uprising in 1808 and started the Peninsular War.

With much of Napoleon's army committed to Spain, a pro-war faction emerged in Austria that included Empress Maria Ludovika of Austria-Este, Foreign Minister Johann Stadion, Archduke John and others.

Austria's French ambassador Klemens von Metternich claimed that Napoleon had only 206,000 troops available and that many were Germans and Poles of doubtful reliability.

Probably encouraged by Grünne, Charles obtained Mayer's dismissal and replacement by Johann von Prochaska.

[1] On 20 April, while the Austrian army maneuvered before the Battle of Eckmühl, Archduke Charles suffered a series of epileptic seizures.

[20] Grünne and Wimpffen were blamed for the defeat by compelling Charles to adopt an overly complex battle plan on the second day.

[21] After another clash in the Battle of Znaim on 11 July 1809, in which the Austrians lost 6,200 casualties, Archduke Charles signed an armistice with Napoleon.

This action enraged Emperor Francis and caused him to demote Charles and dismiss the unpopular Grünne.