One of the confusing aspects of fifteenth century English history is that there were several men called 'Somerset' who pop up, and usually authors do not make full distinction between the individuals. I am going to try to clarify who the various 'Somersets' were, and how they related to one another.
The first Somerset was of course John Beaufort, eldest child of John of Gaunt and Katherine Swynford, legitimised towards the end of Richard II's reign. Briefly promoted to Marquis of Dorset, he lost this title in late 1399, but remained Earl of Somerset and gradually won the favour of his half-brother Henry IV. He married Margaret Holland, sister of (among others) Joanne, Duchess of York, Alianore Countess of March and Edmund, Earl of Kent. John died in 1410, and was succeeded by his son, Henry, who unfortunately died in 1418 before he could fall out with the Yorkists.
This brings us to the first of the Somersets to be a Yorkist bogey-man: John, third Earl of Somerset, known to his friends as 'Lucky'. Not really, but see below.
He had barely succeeded to the title before he was captured at the battle of Bauge (1421). After which he was kept a prisoner of the French for seventeen years. Longer than any other English noble they captured. Albeit it was scarcely a case of durance vile, as he was able to sire an illegitimate daughter, Tacyn (nice name, must use that in a novel sometime) who eventually married Lord Grey of Wilton.
Anyway, after a great deal of negotiation, Somerset was released in 1438 in return for a ransom of £24,000 - which was a lot. This encumbered him financially for the rest of his life. The basis of the Beaufort patrimony was a handful of manors bought by John of Gaunt, with sundry bits and pieces added on by Henry IV, notably an annuity of £1000 a year at the exchequer. (Fine when the exchequer had money in it, by no means to be counted upon in Lancastrian England.) To this can be added his mother's share of the Holland (Kent) lands, worth around £600 a year.
In 1442 John married Margaret Beauchamp of Bletsoe. Her family were (distantly) related to the Beauchamps of Warwick but were more of the status of wealthy gentry than nobility. She was the widow of Oliver St John, by whom she had had six children. This was a perfectly respectable marriage but not a brilliant one for someone of John Beaufort's status - his younger brother had married a mainstream Beauchamp, daughter of the Earl of Warwick!
Apparently Somerset was already in poor health, but in 1443 he was given a chance to redeem his messy financial situation by leading a major English expedition into France. As part of the deal he was given lands of the lordship of Kendal, and other bits and pieces to the value of 600 marks - about £400 - and made Duke of Somerset.
John Beaufort's appointment annoyed the Duke of York immensely, as his commission clashed somewhat with York's existing appointment in France. More to the point, Somerset's expedition was a disaster. Among other things, by holding a Breton town to ransom, it came close to bringing neutral Brittany into the war on the French side. The campaign cost £26,000 and gained nothing.
When Somerset returned to England it was to face Henry VI's anger. Yes, that's right, Henry VI was angry with him! He banished Beaufort from court, and Somerset's financial dealings became subject of an official enquiry.
Somerset went off into the country and died. Some believed he had killed himself. His funeral, at Wimborne Minster, was extremely modest.
Part of his lands reverted to the crown, and his only child, Margaret Beaufort, inherited the lion's share of what was left. The ducal title, and the remnants of the property, passed to John Beaufort's younger brother, Edmund, who will be the subject of the next post.
Source: the most useful source for this has been The King's Mother by Michael K Jones and Malcolm G Underwood.