His military and administrative reforms in the French state were an important factor in assuring the final defeat of the English in the Hundred Years' War.

Arthur was a younger son of Duke John IV and his third wife Joanna of Navarre, and so a member of the Ducal House of Montfort.

He profited by his position at court to obtain the lieutenancy of the Bastille, the governorship of the duchy of Nemours, and the confiscated territories of Jean Larchevêque, seigneur of Parthenay.

However, as the English refused to give him a high command[3] he subsequently returned to the allegiance of the Dauphin in 1424, and was made Constable of France with support from Yolande of Aragon in 1425.

[4] Arthur now persuaded his brother, John V, Duke of Brittany, to conclude the treaty of Saumur with Charles VII of France (7 October 1425).

The peace concluded between John and the English in September 1427, alongside his tenacity and bad temper, led to his expulsion from the court, where Georges de la Trémoille, whom he himself had recommended to the king, remained supreme for six years, during which Arthur tried in vain to overthrow him.

[5] By 1435 he had regained his influence at the French court and then helped arrange the Treaty of Arras between Charles VII and Philip III, Duke of Burgundy.

In alliance with his nephew, the duke of Brittany, he reconquered, during September and October 1449, nearly all the Cotentin; and after the battle of Formigny he recovered for France the whole of Normandy, which for the next six or seven years it was his task to defend from English attacks.

On the death of his nephew Peter II, on 22 September 1457, he became duke of Brittany, and though retaining his office of constable of France, he refused, like his predecessors, to do homage to the French king for his duchy.