The slippery Thomas Stanley (1435-1504) in an engraving from 1838. He was ostensibly a Yorkist, loyal to Edward IV and then Richard III. But he was married to Margaret Beaufort, making him the stepfather of Henry Tudor, the Lancastrian heir. At the battle of Bosworth, Stanley kept his men off to the side, safely out of the action. At the last moment, he betrayed Richard and supported his stepson. It was widely thought at the time that this tipped the balance in Henry's favour. Treachery!!