During the wars of his elder brother Henry V in France, Clarence fought in both the Siege of Caen and the Siege of Rouen (29 July 1418 – 19 January 1419), where he commanded the besieging force.
After Henry had negotiated the Treaty of Troyes, in which he became heir to the French throne, the king returned to England with his new wife Catherine.
The Dauphin, the disinherited former heir, refused to accept the situation and organised continuing resistance, aided by a Scottish army led by John Stewart, Earl of Buchan.

Following the King's instructions, Clarence led 4,000 men in raids through the Anjou and Maine.
 This chevauchée met with little resistance, and by Good Friday, 21 March 1421, the English army had made camp near the little town of Vieil-Baugé.
The Franco-Scots army of about 5,000 also arrived in the Vieil-Baugé area to block the English army's progress; it was commanded by the Earl of Buchan and the new Constable of France, the Sieur de Lafayette; however, the English forces were dispersed, and, significantly, many of the English archers had ridden off in search of plunder or forage.
On Easter Saturday, one of these foraging groups captured a Scots man-at-arms whom they brought before the Duke of Clarence.
Clarence was keen to engage the enemy; however, he had a problem: the following day was Easter Sunday, one of the most holy days in the Christian calendar, when a battle would be unthinkable.
A two-day delay was also deemed as out of the question.
According to the chronicles of Walter Bower, both commanders agreed a brief truce to celebrate Easter, but then joined battle that day.

Perhaps underestimating the size of the Franco-Scottish army, Clarence decided to launch a surprise cavalry-led attack rather than use his archers against the enemy.
With only about 1,500 men-at-arms available, and virtually no archers, he charged the Franco-Scottish lines.
The shock temporarily disordered the Franco-Scots, but soon Clarence and his knights were overwhelmed.
Clarence was unhorsed by a Scottish knight, Sir John Carmichael, and finished off on the ground by Sir Alexander Buchanan, probably with a mace.
During the wars of his elder brother Henry V in France, Clarence fought in both the Siege of Caen and the Siege of Rouen (29 July 1418 – 19 January 1419), where he commanded the besieging force. After Henry had negotiated the Treaty of Troyes, in which he became heir to the French throne, the king returned to England with his new wife Catherine. The Dauphin, the disinherited former heir, refused to accept the situation and organised continuing resistance, aided by a Scottish army led by John Stewart, Earl of Buchan.
Following the King's instructions, Clarence led 4,000 men in raids through the Anjou and Maine.  This chevauchée met with little resistance, and by Good Friday, 21 March 1421, the English army had made camp near the little town of Vieil-Baugé. The Franco-Scots army of about 5,000 also arrived in the Vieil-Baugé area to block the English army's progress; it was commanded by the Earl of Buchan and the new Constable of France, the Sieur de Lafayette; however, the English forces were dispersed, and, significantly, many of the English archers had ridden off in search of plunder or forage. On Easter Saturday, one of these foraging groups captured a Scots man-at-arms whom they brought before the Duke of Clarence. Clarence was keen to engage the enemy; however, he had a problem: the following day was Easter Sunday, one of the most holy days in the Christian calendar, when a battle would be unthinkable. A two-day delay was also deemed as out of the question. According to the chronicles of Walter Bower, both commanders agreed a brief truce to celebrate Easter, but then joined battle that day. 
Perhaps underestimating the size of the Franco-Scottish army, Clarence decided to launch a surprise cavalry-led attack rather than use his archers against the enemy. With only about 1,500 men-at-arms available, and virtually no archers, he charged the Franco-Scottish lines. The shock temporarily disordered the Franco-Scots, but soon Clarence and his knights were overwhelmed. Clarence was unhorsed by a Scottish knight, Sir John Carmichael, and finished off on the ground by Sir Alexander Buchanan, probably with a mace.
Thomas of Lancaster, 1st Duke of Clarence