Henry Tudor’s hereditary claim was slimmer than that of any English King since William the Conqueror, but he was in fact the great-great-grandson of John of Gaunt, who was a son of Edward III and the founder of the House of Lancaster. He was descended from John through an illegitimate son who (along with his siblings) was retroactively legitimized but explicitly barred from succession to the throne, but when the legitimate Lancastrian line became extinct after the battle of Tewkesbury, Lancastrian partisans put their support behind him. He stayed out of the country and kept a fairly low profile during the rest of the reign of Edward IV (of the House of York), but when Edward died in 1483 and was succeeded (after a short reign of his son as Edward V) by his brother, Richard III, Henry or his supporters took the opportunity to stir up discontent against Richard, and in the summer of 1485, Henry invaded England. Their armies met on Bosworth Field, Richard's army was just under 12,000 strong, but 4,000 of his troops were commanded by the Stanley brothers, whose loyalty was suspect. Henry had only 5,000 troops. During the battle both the Stanleys changed allegiance to Henry, swinging the numerical advantage to his favour.
The battle was fought on and around Ambion Hill, close to Sutton Cheney, and lasted only two hours. Richard had the better position, but did not take advantage by attacking Oxford while he was still deploying his troops. This allowed Oxford to launch the first attack and the Duke of Norfolk, who was commanding Richard's forward battle division, was soon killed. For the first hour, the fighting was evenly matched, but Richard lost the battle through the treachery of the the Stanleys, who deserted his cause. Even more damaging was of the Earl of Northumberland's failure to bring Richard's reserves into action when he saw the Stanleys go over to the