After Constance's death, Conan supported her son Arthur I, in his fight against his uncle John Lackland for the Crown of England.
Conan was imprisoned in the castle of Chinon and was released in June 1205 by the French King Philip Augustus.
Conan was the guardian of his nephew by marriage, young Henry II of Avaugour, together with his brother-in-law Geslin of Penthièvre.
Conan the Short then took part to the revolt led by many Breton lords, which was quelled at the Battle of Châteaubriant in 1222.
The lords had accept Peter, who had married Constance's daughter Alix of Thouars, as the new Duke of Brittany.