His alternative regnal name, John VI, as he is known traditionally in old English sources, comes from English partisan accounting as to who was the rightful duke of Brittany during the War of the Breton Succession (1341–65), which had preceded the rule of his father. Although he faced problems which had lingered from it, his rule as duke was mostly unchallenged. Without significant internal and foreign threats, John V reinforced ducal authority, reformed the military, constructed a coherent method of taxation, and established diplomatic and trade contacts with most of Western Europe.