[3] In the War of the Polish Succession he commanded a corps under the orders of Marshal Berwick, capturing Trier and Traben-Trarbach and taking part in the Siege of Philippsburg in 1734.

In 1741 he was sent on diplomatic mission to Frankfurt, Germany, as French Plenipotentiary to carry out, in the interests of France, a grand scheme of political reorganisation in the moribund empire, and especially to obtain the election of Charles Albert, Elector of Bavaria as emperor.

The long tradition of Franco-Austrian rivalry had crystallised around Belle-Isle, who had emerged as the leader of the bellicose bloc of French policy makers towards the House of Austria.

[4] In the eventful year of 1741, he was at the masthead of French interventionist policy in Germany—characterised by scholar Richard Lodge as a "scheme for the humiliation of the House of Austria"[5]—and of the beginnings of the War of the Austrian Succession.

[6] Belle-Isle was named Maréchal de France in 1741 and received control of a large army, with which it is said that he promised to make peace in three months under the walls of Vienna.

In ten days he led 14,000 men—5000 men stayed in the city under the command of François de Chevert[8]—into and across the Upper Palatine Forest whilst being harassed by the enemy's light cavalry and suffering great hardships.

[9] The campaign, however, had discredited Belle-Isle; he was ridiculed in Paris and Fleury is said to have turned against him, and to complete his misfortunes, he was taken prisoner by British forces while going from Cassel to Berlin through Hanover.

However, the elder Belle-Isle still managed to repel a follow-up invasion of the Provence by Austrian and Italian forces and pushed the fighting back into the plain of Lombardy.

Belle-Isle awarded royal architect Jacques-François Blondel for the embellishment of the town square and the construction of the city hall, the parliament, and the guardhouse lodging.

He also decided the edification of the royal Governor and Intendant palaces and the opera house of Metz, describing it as "one of the most beautiful of France's opera-theatres" at his time[citation needed].