Plantagenet Birth Control
On this day, in 1399, the Duke of Lancaster decided to promote himself King of England (and an unwilling Wales). To become Henry IV, however, he first had to oust his cousin Richard II. But no one except Richard seemed to mind.
Richard II had the rare distinction of being both unethical and incompetent. The progressive nobles despised his blundering misrule. The conservative lords loathed his personal conduct; Richard was a bit too poetic and he practiced hygiene before it was fashionable. Someone was going to murder him, and the reformist cousin Henry struck first.
But then the conservative cousins in the dynasty, pretending to avenge Plantagenet family values, tried to wrest the throne for themselves. This struggle lasted for 85 years and 8 Shakespeare plays.
By 1485, the English throne had become quite democratic. Anyone could seize it. The surviving claimant Henry VII based his right to the throne on being the illegitimate half-second cousin, once removed, of Henry VI. (He was also the illegitimate half-nephew but that family connection was less prestigious.)