Edmund Beaufort, fourth duke of Somerset (1439-1471) was a key Lancastrian leader during their period in exile after Edward IV's victory in the first phase of the Wars of the Roses, and was executed after defeat at the battle of Tewkesbury had ended the short-lived Lancastrian revival of 1470-71.
Edmund Beaufort was the second son of Edmund Beaufort, second duke of Somerset (1406-1455), and the brother of Henry Beaufort, third duke of Somerset (1436-1464). His father was killed at the First Battle of St. Albans in 1455, and Henry Beaufort became a fierce enemy of the Yorkists. The younger Edmund wasn't involved in the early fighting in the Wars of the Roses, or in the campaigns of 1459-60. In 1460 he was commander on the Isle of Wight, but was captured by Geoffrey Gate, a retainer of the earl of Warwick. He was imprisoned in Carisbrooke Castle, and then in the Tower of London.
By the end of 1462 the third duke had begun to lose his enthusiasm for the Lancastrian cause, and after he surrendered Bamburgh Castle in late December Edward IV attempted to win his loyalty. As part of this effort, early in the summer of 1463 Edmund Beaufort was released. Edward's attempts failed. Somerset soon returned to the Lancastrians and led a short-lived Lancastrian revival in the north-east of England, while Edmund joined Queen Margaret in Scotland. Somerset's successes ended with two defeats, at Hedgely Moor (25 April 1464) and Hexham (15 May 1464). Somerset was captured at Hexham and executed after the battle. In 1465 Parliament passed a posthumous act of attainder against him, stripping him and his heirs of their estates and titles.
By this point Edmund Beaufort was in exile in France. He was in Paris by October 1464, and had joined the court in exile at Koeur near St Mihiel-en-Bar by the end of the year. He ignored the act of attainder and styled himself as duke of Somerset. His title was recognised by the Lancastrian court in exile, ignored by the Yorkist court in England, but posthumously recognised by the Tudors.
Now he was in exile Somerset began to build a military reputation. He formed a close friendship with Charles, count of Charolais, the heir to the duchy of Burgundy. He fought with Charolais during the Montlhéry campaign of 1465, and during the Liege campaign of 1467. In the same year Charles became Duke of Burgundy, and Somerset was a regular attendee at his court. A group of exiled Lancastrians began to congregate around the Burgundian court, even after the Duke married Edward IV's sister Margaret in 1468. The Duke of Burgundy's main aim was to prevent the earl of Warwick from gaining political power in England - Warwick was known to favour a French alliance that might threaten the existence of Burgundy.
In 1469 the apparently stable Yorkist regime began to shake itself apart. The earl of Warwick wasn't happy with Edward IV's independent attitude or his perceived lack of rewards, and started to plot with Edward's brother Clarence. After a brief campaign Edward was taken prisoner, and Warwick attempted to rule through a captive king. This worried Burgundy, and he gave Somerset permission to prepare to raise an invasion force, but by the end of 1469 Edward had regained control. Early in 1470 Warwick attempted to regain control himself by triggering revolts in Lincolnshire and Yorkshire, but the plan failed and Warwick was forced into exile. This time he was unable to get access to Calais, and after some naval adventures was forced into exile in France.
Warwick's only option now was to ally with Queen Margaret. After some tricky negotiations, in early August 1470 the two enemies agreed to work together. Warwick agreed to restore Henry VI, with Prince Edward as his heir, followed by Clarence. Warwick was to become the king's lieutenant. Anne Neville married Prince Edward in an attempt to cement the obviously shaky alliance. Queen Margaret didn't trust Warwick enough to allow him to take Prince Edward with him when he attempted to depose Edward IV, a mark of the weakness in the new alliance. Somerset was offered the restoration of his estates, but he was wary about the prospect of an anti-Burgundian alliance between Warwick and Louis XI of France, and didn’t take part in the campaign that saw Edward IV forced into exile and Henry VI's 'readeption government' take power.
Once Henry was back on the throne, Somerset was willing to offer his support. He left Burgundy in November 1470 and was soon back in England. He argued against the French alliance during a personal audience with Henry VI on 14 February, but on the following day Warwick declared war on Burgundy. This created a split within the Lancastrian leadership, one that would soon have fatal consequences.
In March 1471 Edward IV landed on the Yorkshire coast. Ironically the Yorkist king wasn't popular in Yorkshire, and he had to trick his way through the county by pretending he was only trying to regain the duchy of York. Warwick failed to intercept Edward in the Midlands and the war developed into a race for London. Somerset and John Courtenay were the senior Lancastrian leaders in London, and they probably had enough men to hold the city long enough for Warwick to arrive and trap Edward between the city and his army. They certainly received orders to defend London, but at about the same time they learnt that Queen Margaret and Prince Edward had finally set sail for England. On 8 April Somerset and Courtenay left London to meet with Queen Margaret, leaving the defences of the city in the hands of George Neville, archbishop of York. This was a poor decision on Somerset's part. Neville made very little effort to defend the city, and Edward was able to enter London on 11 April. There he found reinforcements and extra guns, and on 14 April 1471 he defeated Warwick at the battle of Barnet.
On the same day Queen Margaret and Prince Edward landed at Weymouth. On 15 April Somerset met them at Cerne Abbey, and convinced the Queen to continue with her invasion. Edward IV was still vulnerable, and the Lancastrians were able to raise quite a powerful army in the south-west. The decisive battle of this phase of the wars came at Tewkesbury on 4 May 1471. Somerset attempted to outflank the Yorkists, and was able to successfully attack Edward's main battle in the flank. This left the Lancastrian centre comparatively weakened and allowed Richard of Gloucester to aid his brother. Somerset's men held on until they were attacked in the flank by a force of 200 men-at-arms that Edward had ordered to watch for an possible ambush. Somerset's battle broke and fled, leaving Edward free to concentrate on Prince Edward. The rest of the Lancastrian army was soon defeated and the young prince was killed. Somerset's brother John was also killed in the battle, but Somerset himself managed to reach sanctuary in Tewkesbury Abbey. Edward ignored his own initial promise to pardon anyone who had fled to the abbey church, dragged Somerset out of sanctuary and after a brief trial had him executed on 6 May. This ended the male line of the Beaufort family, and with it the consistent threat they had posted to the House of York.