His request was gladly granted, for the presence of the Duc de Bouillon's brother in the French army was in itself some guarantee of the duke's fidelity to his engagements with France, and Turenne was at once appointed to the colonelcy of a regiment.

He devoted himself as assiduously to his work as he had done in Holland, and it was not long before his regiment gained the reputation of being the best disciplined in the king's service.

He took part in a short expedition in 1630, but there was on that occasion no fighting, and he first saw real service under Marshal de la Force in 1634.

After the siege of La Motte, the success of which was due to the storming of the breach by Turenne and his regiment, and for which exploit he was promoted to the rank of Marechal de Camp, a rank equivalent to that of major general, he took part in several sieges, until Lorraine was completely conquered and its duke driven to abdicate and retire to Austria.
The battle of Nordlingen showed Richelieu that if France did not resolutely enter into the conflict the Austrians would become absolute masters of all Germany.

He at once signed a treaty with the Swedes, agreeing to grant them large subsidies to carry on the war.