From 1370 to his death, he was Constable of France for King Charles V. Well known for his Fabian strategy, he took part in seven pitched battles and won the five in which he held command.

[4] Bertrand's family may have claimed descent from Aquin, the legendary Muslim king of Bougie in Africa (Viking in effect, the legend conflates Saracens and Arabs with Normans and places Aiquin's origins in the north country) a conceit derived from the Roman d'Aquin, a thirteenth-century French chanson de geste from Brittany.

In 1366, du Guesclin, with Guillaume Boitel, his faithful companion, leader of his vanguard, captured many fortresses (Magallón, Briviesca and finally the capital Burgos).

After Henry's coronation at Burgos, he proclaimed Bertrand his successor as Count of Trastámara and had him crowned as King of Granada, although that kingdom was yet to be reconquered from the Nasrids.

[9] While turning Pedro down, du Guesclin is claimed to have said "Ni quito ni pongo rey, pero ayudo a mi señor" (I neither remove nor put a King, but I do help my Sire), which has since that moment become a common phrase in Spanish, to be used by anyone of lesser rank who does what he is ordered or expected to do, avoiding any concern about the justice or injustice of such action, and declining any responsibility.

War with England was renewed in 1369, and Du Guesclin was recalled from Castile in 1370 by Charles V, who had decided to make him Constable of France, the country's chief military leader.

By tradition, this post was always given to a great nobleman, not to someone like the comparatively low-born Du Guesclin, but Charles needed someone who was an outstanding professional soldier.

He immediately defeated the remnant of an English army, which had been led by Robert Knolles until his retreat at Du Guesclin's coming, at the Battle of Pontvallain, and then reconquered Poitou and Saintonge, forcing the Black Prince to leave France.

An able tactician and a loyal and disciplined warrior, Du Guesclin had reconquered much of France from the English when he died of illness at Châteauneuf-de-Randon while on a military expedition in Languedoc in 1380.

[12] Bertrand du Guesclin appears as a character in Arthur Conan Doyle's 1891 historical novel The White Company, set in 1366.

He is also a major character in the trilogy of Dutch historical youth novels Geef me de Ruimte, by Thea Beckman, the first part of which was published in 1976.