At sixteen years of age, he escaped from the paternal mansion, went to Rennes, entered upon a course of adventures, quarrels, challenges, and tourneys, in which he distinguished himself by his strength, his valor, and likewise his sense of honor.

He joined the cause of Charles of Blois against John of Montfort, when the two were claimants for the duchy of Brittany; but at the end of thirty years, "neither the good of him, nor his prowess, were as yet greatly renowned," says Froissart, "save amongst the knights who were about him in the country of Brittany." But Charles V., at that time regent, had taken notice of him in 1359, at the siege of Melun, where Du Guesclin had for the first time borne arms in the service of France.

When, in 1364, Charles became king, he said to Boucicaut, marshal of France, "Boucicaut, get you hence, with such men as you have, and ride towards Normandy; you will there find Sir Bertrand du Guesclin , hold yourselves in readiness, I pray you, you and he, to recover from the King of Navarre the town of Mantes, which would make us masters of the River Seine." "Right willingly, sir," answered Boucicaut; and a few weeks afterwards, on the 7th of April, 1364, Boucicaut, by stratagem, entered Mantes with his troop, and Du Guesclin, coming up suddenly with his, dashed into the town at a gallop, shouting, "St.Yves! Guesclin! death, death to all Navarrese!" The two warriors did the same next day at the gates of Meulan, three leagues from Mantes.

"Thus were the two cities taken, whereat King Charles V.was very joyous when he heard the news; and the King of Navarre was very wroth, for he set down as great hurt the loss of Mantes and of Meulan, which made a mighty fine entrance for him into France." It was at Rheims, during the ceremony of his coronation, that Charles V.
heard of his two officers' success.

The war thus begun against the King of Navarre was hotly prosecuted on both sides.