After the death of his father, he participated in the military campaigns of his brother in France during the Hundred Years' War. Left in charge of English forces in France when Henry returned temporarily to England after his marriage to Catherine of Valois, Thomas led the English in their disastrous defeat at the hands of a mainly Scottish force that came to the aid of the French at the Battle of Baugé. In a rash attack, he and his leading knights were surrounded, and Thomas was killed.