He entered into a war with Martel, who pretended to the government of the Saintonge. On 20 September 1034, he was captured in the field at Moncontour, near Saint-Jouin-de-Marnes. He was freed in 1036, after nearly three years imprisonment, only by ceding the cities of Saintes and Bordeaux.
He immediately reopened the war, but was defeated again and had to cede the isle of Oléron.
William married Eustachie of Montreuil but had no known descendants. He reformed the administration of Poitiers by naming a provost and died there, being buried at Maillezais, being succeeded by his half-brother Odo.