[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.

When peace was made in 1736, Louis XV gave Belle-Isle the governments of three important fortresses, Metz, Toul and Verdun, offices that he would hold until his death.

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.

This was at least partly because Frederick the Great, having already accomplished his aims, pulled Prussia out of the war prematurely, leaving Belle-Isle at the mercy of a much larger Austrian army.

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.

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].