French campaign and imprisonment
He fought in the 1419 French campaigns of his cousin King Henry V. In 1421 he accompanied his step-father Thomas of Lancaster, Duke of Clarence (the king's younger brother) on a campaign in Anjou, France. Thomas was killed at the Battle of Baugé (22 March 1421), while Somerset and his younger brother were captured and imprisoned for 17 years. On 25 March 1425 he came into his majority, but his paternal estates had to be managed by his mother for the next thirteen years of his imprisonment. He remained imprisoned until 1438 and having been ransomed, became one of the leading English commanders in France.
He was appointed Admiral of the Sea to the army commander John Talbot, 1st Earl of Shrewsbury, who from August 1440 besieged Harfleur, which had been in French hands for five months. King Charles VII of France sent a large army under Richemont to break the siege. The English dug a double ditch rampart with only 1,000 men, while Somerset's squadron prevented a French landing by sea, using archers to pick off the enemy at short range. Having been thus frustrated, the French withdrew to Paris and the siege was lifted. The town surrendered to the English and was reoccupied.
The Duke of York was incensed that Somerset’s uncle Cardinal Henry Beaufort had advised the king to sue for peace. Somerset also advised King Henry that peace was humanitarian and that the king of France was determined to seize Pontoise. By the time York arrived at the Normandy campaign in 1441, Somerset had resigned. The fall of Pontoise to Charles, Duke of Orléans in September 1441 weakened English garrisons, and in Gascony the situation was even worse. The Beauforts sent Sir Edward Hull, who arrived at Bordeaux on 22 October 1442, to inform York that a huge army would arrive, commanded by Somerset. York was ordered to fortify Rouen. Just as the king and Dauphin of France were threatening Bordeaux and Aquitaine and seized the town of Dax, Somerset dithered. York was held back as Guyenne was being lost.
