John of Gaunt, third son of Edward III, was the Duke of Lancaster, and his illegitimate children, the Beauforts, were barred from the throne by his legitimate, firstborn son, Henry IV. Clearly the latter wasn’t having any baseborn relative wearing the crown. Nevertheless, we eventually ended up with a Beaufort king, who claimed to be the last Lancastrian heir. He wasn’t.
Explanation is needed to sort out the intricacies of it all. The Beauforts were not true Lancastrians at all, because though they descended from John of Gaunt, Duke of Lancaster, Edward III’s third son, it was a fact that Gaunt only had the title because of his first marriage, to Blanche of Lancaster. So Blanche’s descendants, Henry IV, Henry V and Henry VI, were proper Lancastrians. The baseborn Beauforts descended from Gaunt’s mistress and eventual third wife, Katherine de Roët. Their eventual legitimisation by the ill-fated true king, Richard II, son of the Black Prince, Edward III’s eldest heir, did not change this. The Beauforts were never true Lancastrians. Without Blanche’s blood, they couldn’t be. (1)
After Henry VI, if the proper Lancastrian line, i.e. from Blanche Lancaster, were to have been continued, it would have been through the Portuguese offspring of Philippa of Lancaster, Gaunt’s elder daughter by Blanche.
Except, of course, that the Lancastrian line had never been the true one in the first place. The House of Lancaster usurped Richard II’s throne and then murdered him. The rightful line after Richard II was that of Lionel, Duke of Clarence, who had been Edward III’s second son.
Gaunt was a hypocrite. He tried his damnedest to persuade Edward III to prevent the throne from ever descending through a woman. This was in order to exclude the descendants of Lionel of Clarence. Lionel left a single daughter, Philippa of Clarence, who married Edmund Mortimer, Earl of March. Their only child, Anne, married Richard of Cambridge, a son of Edmund of Langley, thus uniting the second and fourth line of descent from Edward III. Thus the true House of York, as we know it, was created.
Of course, as far as Gaunt was concerned, staking a claim to the throne of Castile through his own second wife, Constance of Castile, was another matter entirely. It was just and noble, and through her he considered himself to be the King of Castile. He even demanded to be known as that. Yet he wanted such claims through the female line to be eliminated in England. Yes, a hypocrite of the highest order.
I can understand Gaunt’s wish to legitimise his children by Katherine, whom he clearly loved. But I cannot forgive his two-faced, underhanded scheming to steal a throne that was not his to steal! His son did steal it—through usurpation and murder, and that’s how we ended up with the three kings of the House of Lancaster, Henry IV, Henry V and Henry VI. But the House of York did ascend the throne eventually, in the form of Edward IV and then Richard III.
Back to Gaunt. In the name of Lancaster, he had raised an army and sailed off to take a (foreign) throne that was occupied by someone else. And he did this through the claims of a woman, no less. Fast forward to the aftermath of the sudden death of the Yorkist king, Edward IV, and we have scheming Margaret Beaufort and her son, Henry Tudor, neither of whom truly represented the Lancastrian line. But they posed as such. Throughout the tragically short reign of Edward’s last brother, Richard III, they plotted against him. Their treachery, in the name of Lancaster, led to Henry’s foreign invasion and Bosworth, where Richard was betrayed and killed.
Henry Tudor promptly stepped up to the throne. Um, perhaps not in the name of Lancaster, more for himself. He was careful to claim victory through conquest, not blood line. Which tells me that he was well aware that his mother’s Beaufort descent was a very doubtful blessing. The Beauforts had been barred from the throne by an only too Lancastrian monarch, Henry IV.
Henry Tudor knew he had defeated and ended the life of the last true King of England. He, like Henry IV before him, was a regicide. (Yes, yes, I am aware that the same charge can be laid at Edward IV’s door, regarding Henry VI, but that is another story entirely.)
So, to sum up. No Lancastrian, of any degree, should ever have been king. From Richard II, the line should have descended through Lionel of Clarence, the Mortimers and York. Richard III did thus descend. The crown of England was his by right of birth. That could never be said of Henry Tudor, whose sole right was based upon foul treachery.
(1) See also: The Lancastrian claim to the throne, Ashdown-Hill, pp.27-38, Ricardian 2003