In 1346 he led a small English force in Brittany in support of John de Montfort's claim on the dukedom. De Montfort was backed by the English throne, whereas his rival, Charles of Blois was backed by the French. On 9 June, Dagworth's force was attacked by Charles' much larger army at Saint-Pol-de-Léon. Though almost surrounded, the longbowmen won the day for the English. The next year, on 20 June, he claimed an even more famous victory at the Battle of La Roche-Derrien, where Charles of Blois was captured.
He was killed in an ambush in 1352 by Bretons unhappy with the English presence.
Sir Thomas came from Bradwell Juxta Coggeshall in Essex. In 1343 he had married Eleanor de Bohun, Countess of Ormonde, the daughter of Humphrey de Bohun, 4th Earl of Hereford and Elizabeth Plantagenet, King Edward II's sister.
- Alison Weir: Britain's Royal Family. A Complete Genealogy. The Bodley Head, London, U.K. 1999, S. 84.
- Burke, John (1831) A general and heraldic dictionary of the peerages of England, Ireland, and Scotland, extinct, dormant, and in abeyance, p. 155
- Turnbull, Stephen. The Book of the Medieval Knight. London: Arms and Armour Press, 1985. ISBN 0-85368-715-3
- thepeerage.com Accessed 22 March 2008
- A History of Dagworth (including the de Dagworth family tree)
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