In service of the French
Most historians argue that Luis returned to France to mainly improve his fortunes. As a cousin of the French King, he entered the French court.
King Philip VI of France invested Luis de la Cerda as Count of Clermont and the first Count of Talmont in 1338/39. The King set up these estates in an area of France immediately bordering the English possessions of the Guyenne in the south west and in the north east, and was particularly exposed.
Accounts show that in 1339 the French Army was garrisoned at his castle of Talmont, to defend Lille He was titled “Sovereign captain” at that stage.
In 1340, Luis received a testimony of royal confidence when he was called on March 13 to fill the post of admiral left vacant by Hue Quiéret, fatally wounded in a naval battle. During the truce that followed, Luis worked to rebuild the navy, with new ships being built and fitted out ships available in all ports of France, while also recruiting crews from maritime populations of the Basque provinces.
During the Breton War of Succession, Luis joined the command of the Duke of Normandy in October 1341, travelled to Angers where a Franco-Breton army was massing, joining the likes of the Viscount de Rohan and lords of Avaugour, Olivier IV de Clisson and Beaumanoir.
During the siege of Nantes where the opposing forces were stationed, the starving civilians seized a convoy of food outside the walls and subsequently were attacked by Luis, pursued to the foot of the walls, where a furious melee ensued. Half of these people found themselves cornered on the ramparts, were killed or taken prisoner.
Luis resigned as admiral in March 1341, no record is known to exist for historians to understand the motives for this. What is known is by spring 1342 he had directly joined the Franco-Breton army of Charles de Blois as a commander.
During the siege of Hennebont, Luis was issued siege weapons by Charles after he had moved on to attack Auray; Luis accelerated operations as quickly as he could, and soon the defenders were open to surrender talks when an English fleet arrived. The besieged exited the town and destroyed one of the siege weapons. In a council with his principal lieutenants, Luis resolved to lift the siege and join the rest of Franco-Breton army attacking Auray.
Luis then moved on Dinan which had no defenses other than the Rance river and palisades. To make the river crossing, Luis ordered rafts be built, but these preparations were moot, for at the end of four days, the defenders killed their own captain and opened the gates.
Luis then crossed the width of Brittany to besiege the commercial town of Guérande. Luis used his Spanish and Genoese mercenaries to attack all sides at once. Without giving quarter to the inhabitants, they engaged in plunder, not even respecting the churches, five of which were desecrated and burned. When Charles heard of these excesses, he ordered Luis to hang 24 principal culprits. Luis still left Guérande, loaded with loot and proceeded to join Charles, still in front of the walls of Auray.
Luis with his mercenaries, then set sail for Quimper and Quimperlé to continue pillaging. As news of the devastation reached the Anglo-Bretons under de Mauny, he left Hennebont for Quimperlé where Luis's forces were still ashore. De Mauny damaged badly-guarded vessels before disembarking his own troops, dividing them into 3 groups and began to search for the looters. His vanguard was repulsed by Luis who, took to the field with one of his nephews, named Alphonse. The local peasantry also joined, and quickly routed Luis's force, killing his nephew.
Luis recuperated at Rennes and after six weeks, rejoined Charles de Blois in a second attempt at capturing Hennebont. The Anglo-Breton defenders, who, aware of his recent defeat, taunted him from the ramparts. According to the chroniclers Jean Le Bel and Froissart, this mocking angered Luis so much that he sought some form of retribution. These chroniclers, state that one day in the tent of Charles de Blois, and in front of other lords, begged him to grant him a grace in reward for his services. Charles did not attach much importance and agreed to this request. Luis apparently then asked to be given two captured Anglo-Breton knights, Jean le Bouteiller and Hubert de Frenay. Luis apparently then added that he wanted them beheaded in front of the city's ramparts. Charles was astonished, telling Luis that these prisoners were Christians, not Moors and that such an act would be dishonorable. Luis refused to change his resolve and threatened to quit the army if he did not obtain satisfaction. Charles gave in and the two knights, were sent for. Luis remained unmalleable to showing mercy and announced that the execution would take place the same day after dinner. The Anglo-Breton defenders apparently had spies in Charles camp and became aware of the situation, resolving to try everything to snatch them back. While Amaury de Clisson simulated a distraction to attract the attention of the besiegers, Mauny took a detour into the now empty camp and recaptured the two knights returning with them. The second siege of Hennebont then continued without any progress; being finally abandoned around the middle of October. Charles then attacked the small town of Jugon, while the rest of the army was garrisoned in Garhaix under the command of Luis.
Luis was finally involved in a naval battle fought in the vicinity of the Island of Guernsey in 1342 between an English fleet, bringing aid to the Anglo-Bretons and a Franco-Breton fleet, consisting mainly of Luis's mercenaries. The fight which started at the end of the afternoon was interrupted by twilight, only resuming again at dawn. A storm that arose during the night forced the adversaries to part ways, but not before the Franco-Bretons had captured four Anglo-Breton ships loaded with provisions and horses. This however did not stop the Anglo-Breton commander Robert III of Artois landing his troops in Brittany. The chroniclers relate that Luis was involved for the rest of the war in seizing English supply ships intended for the Anglo-Breton force besieging Vannes, and surprising an enemy fleet anchored not far from there, to seizing four ships loaded with supplies, three of which were sunk. Luis left the War of Breton Succession very rich as a result of the huge loot and plunder captured.
During the War in the Guyenne, in 1346, Luis resumed service for Philippe de Valois. The chroniclers state that he participated in a campaign led by the Duke of Normandy against the English commanded by the Earl of Derby during the first half of that year. The season's campaign ended in failure in front of the stronghold of Aiguillon. According to Froissart, Luis later distinguished himself by finally braking through the walls of this fortress.