Nicknamed very early on "The Black Dog of Brittany" due to his ugliness and bravery, Bertrand Du Guesclin, this eldest son of Robert Du Guesclin, Lord of La Motte-Broons (near Dinan), first took arms in 1342 at the side of Charles de Blois in the War of Succession in Brittany.

In 1357, he entered the service of Charles V, whose father John II (The Good) was a prisoner of the English. On the death of John II in 1364, Du Guesclin celebrated the accession of Charles V by winning the Battle of Cocherel against Charles of Navarre. However, defeated at the Battle of Auray, he was taken hostage by Chandos, the head of the English army (September 1364). Charles V paid a ransom of 100,000 pounds for his release.

Then he made Du Guesclin responsible for leading an expedition to support Henry of Trastámara on the throne of Castile and thereby to distance France from the dangerous Great Companies. But Du Guesclin was defeated by the Anglo-Castilian army in Najera (1367). The King of France paid his ransom once again. Du Guesclin nevertheless managed to re-establish Henry of Trastámara (Henry II of Castile) in 1369. Then appointed a Constable by the King (1370), Du Guesclin spent ten years chasing the English from Poitou, Normandy, Guyenne and Saintonge and reversed the military situation in favour of France. On his death, exceptionally, he was buried in the Royal necropolis of Saint-Denis, next to Charles V.