With the Duchy of Brittany and the Kingdom of England Jean fought under the command of the English
Edward the Black Prince, participating in the
Battle of Nájera (1367), against the French led by
Bertrand du Guesclin. Although the English won the battle, the Black Prince went bankrupt. He was one of the signatories of the second
Treaty of Guérande in 1381 as a representative for the
Duke of Brittany John IV. The Duke had restored his rights and agreed to make a small tribute to the King Charles VI. Furthermore, Brittany would stay neutral in continuing military conflicts between France and England.
With the Kingdom of France Jean then offered his services to France. In 1382 he fought at the
Battle of Roosebeke. In 1397 De Rieux was appointed as Marshal of France. In 1405 he was sent to
Wales to help the Welsh with the
Glyndŵr Rising. He had left
Brest in July with more than 3,000
knights and
men-at-arms. Unfortunately, the army had not been provided with sufficient fresh water and many warhorses had died. He did though bring modern
siege equipment. Joined by
Owain Glyndŵr's forces the French marched inland and took the town of
Haverfordwest but failed to take the castle. He then marched on England, but the result of this expedition was not successful. His age and the injuries that he had received over the years eventually prevented him from performing his duties. He was, at his request, relieved from his duties at 12 August 1417. He died the same year in his castle at
Rochefort, where he was buried. ==References==