Spain, during the burst of confidence after its unexpected great victory at Nordlingen in 1634, made plans for major operations in Germany to end the war against the Protestants there and in the Netherlands; these plans included Thomas leading an army in Westphalia, under the overall command of the Cardinal-Infante Ferdinand, brother of Philip IV. Nothing came of this, but in 1635, when France declared war on Spain (Franco-Spanish war of 1635-59), Thomas served under Ferdinand in the Spanish Netherlands: he was given command of a small army (variously given as 8,500 or 13,000) sent against French forces that had advanced into Luxemburg, his orders either to observe them or to prevent them from joining up with a Dutch army. On 22 May 1635 at Les Avins, south of Huy, in what was then the bishopric of Liège, he was defeated by the French army commanded by Breezé and Chatillon. He managed to rally the remnants at Namur, then retreated before the numerically-superior French and Dutch forces; and he probably served the rest of the campaign with Ferdinand. Late in the year, the refugee Charles IV, Duke of Lorraine arrived in Brussels and met Thomas; they may have formed a joint court, and Thomas certainly participated in jousts organised by the Duke. (In this Franco-Spanish war, Piedmont was reluctantly dragged into the fighting alongside the French, though initially it avoided a full declaration of war; consequently, Thomas was technically fighting against his own homeland.)