Sir William Stanley (c.1435-1495) was a supporter of the House of York who is best known for siding against Richard III, the last Yorkist king, at the battle of Bosworth in 1485, effectively deciding the outcome of the battle.
William was the second son of Thomas Stanley, first Lord Stanley and the brother of Thomas Stanley, first Lord Stanley, the future earl of Derby. His brother married Eleanor, daughter of Richard Neville, earl of Salisbury, in the late 1450s, but despite that family connection tended to stay neutral during the early phases of the Wars of the Roses.
In contrast William was a consistent supporter of the House of York, at least until the reign of Richard III. The second phase of the Wars began in 1459 when the Lancastrians prepared to strike against the Yorkist lords. In response the Yorkists mobilized. Salisbury raised an army in the north of England, and William joined this army. Salisbury then had to try and get past the Lancastrian forces in the Cheshire-Shropshire area. On 23 September 1459 he defeated one of the Lancastrian armies in the area at Blore Heath. Lord Stanley was nearby with 2,000 men but didn't take part. In contrast William fought in the battle, and would be attainted as a traitor in the Coventry parliament that followed the Lancastrian victory.
Although Salisbury had managed to elude the Lancastrians at Blore Heath, the Yorkists were still badly outnumbered and after a standoff at Ludlow their leaders fled into exile (battle of Ludford Bridge, 12-13 October 1460). William probably accompanied them into exile, although he may have hidden in his brother's domain in the north-west.
William took part in the great Yorkist revival of 1460-61, and fought for Edward IV at Towton in March 1461. On 1 May he was rewarded with the posts of chamberlain of Chester, constable of Flint Castle and sheriff of Flintshire. He held these posts for the rest of his life. He was knighted in July. He served under the earl of Warwick during the siege of Alnwick (1462) and fought at Hexham in 1464. That battle destroyed a Lancastrian enclave in the north, and in the aftermath Sir William was granted the lands forfeited by John, Lord Clifford, who had been killed at Ferrybridge just before the battle of Towton.
In 1469 Warwick turned against Edward IV, but despite the family connection to Warwick Sir William stayed loyal to Edward. He refused to help Warwick in his revolts of 1469 and 1470, and when Edward IV returned from exile in March 1471 Sir William was one of the first to join him, bringing 300 men to Edward's tiny army at Nottingham.
In the 1470s Sir William was a key member of the Yorkist regime. Edward's eldest son became Prince of Wales, and Sir William was steward of the prince's household. He and his brother were both involved in territorial disputes with Richard, duke of Gloucester, and managed to keep him out of their heartland in Lancashire and Cheshire. Richard may have come to terms with this setback, for in 1475 he swapped Chirk for Skipton, which was much closer to his main centre of power.
In 1483 Edward IV died. Sir William was a natural supporter of his son Edward V, but he was unable to protect the young king from his uncle Richard, who swiftly usurped the throne as Richard III. He helped suppress Buckingham's rebellion in October 1483, and Richard made an effort to win over the powerful Stanleys. Sir William was made chief justice of north Wales, and was given Buckingham's lordship of Thornbury (in 1484 he swapped Thornbury for Holt, Bromfield and Yale, consolidating his power in the north-west).
Richard failed to win over the Stanleys. When Henry Tudor invaded in 1485 Sir William visited him at Stafford on 17 August and presumably promised assistance. The Stanleys had one big problem - Lord Stanley's son Lord Strange was being held hostage by Richard III, and so they couldn't risk openly supporting Henry. Even after Richard condemned Sir William as a traitor he still didn't openly join Henry.
Sir William played a vital role in the battle of Bosworth Field (22 August 1485). While the main bodies of both armies were engaged in battle Richard noticed Henry Tudor was exposed. Richard led his household troops in an attack on Henry, and for some Henry was in danger. At this point Sir William finally led his men into the battle on Henry's side. Richard was killed and with his death the heart went of his supporters.
Sir William was rewarded for his actions by being made chamberlain of the royal household. All of Richard's grants to him were confirmed and he became a powerful and wealthy figure. He supported Henry VII during Lambert Simnel's revolt of 1487 and the Stanley forces fought at the battle of Stoke, but at some point after this Sir William seems ot have become discontented with the new regime.
Late in 1494 Sir William was suddenly arrested and charged with being involved with Perkin Warbeck's conspiracy. He was charged with having been in contact with Warbeck and sending a mission to him to discover if the pretender was actually Edward IVs son Richard. He was tried on charges of treason on 6-7 February 1495 (with his brother, now earl of Derby and constable of England) presiding, and was convicted. Sir William Stanley was beheaded at the Tower of London on 16 February 1495.