Battle of Bosworth Field (22 August 1485) took place because at that time Henry Tudor was able to mount a serious challenge to the position of King Richard III. He was able to do so because during the preceding two years Richard's position had been weakened by his own acts and by rebellion and discontent among the English nobility, and because Henry had been able to secure his own place as the head of the Lancastrian tendency and the only credible challenger to Richard's throne. This was despite the very real weakness of Henry Tudor's claim. Henry's royal blood was real enough but illegitimate, and his claim to the throne was not a particularly strong one; it was based on his descent from the Beaufort line, illegitimate offspring of John of Gaunt's union with his mistress Kathryn Swinford. John of Gaunt was son of Edward III and held the title Duke of Lancaster; his eldest legitimate son became the first Lancastrian king of England, Henry IV. Margaret Beaufort, granddaughter of John of Gaunt, was married in 1453 to Edmund Tudor, Earl of Richmond, half-brother of King Henry VI. When relations between the Lancastrian court and the Duke of York broke down in the late 1460s the sons of Edmund Tudor, Owen and Jasper, sided not with the King but with York. When Henry VI and his son Edward died in 1471 Henry became de facto head of the House of Lancaster.

The revolts against Richard that took place in 1483 had the dual effect of weakening Richard further and strengthening Henry, and the latter was able to prepare militarily over the next two years to challenge Richard for the throne.

The fact that Henry's place of exile was Brittany, a state hostile to Richard's England, made it more difficult for Richard to undermine his rival.

Many notables with a shared interest in removing Richard from power gravitated to Henry's following during 1484 and early 1485 until he had a retinue of some 500 individuals, ranging from die-hard Lancastrian loyalists to those who had fallen foul of Richard in some way and sought revenge. The clear strength of Henry's following convinced the King of France to give him the financial aid he had long sought, making invasion a realistic possibility. Meanwhile Richard's son, Prince Edward, died in 1484 and his wife, Anne Neville, died in early 1485, depriving him of both his heir and his means of producing another. This raised the issue of what would happen if Richard died childless; the threat of a further disputed succession and perhaps another bout of civil war must have loomed large in the minds of many of Richard's former supporters and made the cause of the young Henry Tudor still more appealing.

Thus, by the spring of 1485 Henry stock was high, and Richard's correspondingly low. The time for invasion and for a decisive meeting on the field of battle had come. That meeting would take place on Bosworth Field on 22 August. Henry's small invasion force landed at Milford Haven in south-west Wales on 7 August 1485. Henry's Welsh descent and conscious identification with Welsh national aspirations was itself an important element in his support-base.

He marched through Wales and into England, gathering support as he did so; Richard, from his base at Nottingham, saw much of his support dwindle and many promises of loyalty come to nothing over the same period. By 21 August the Tudor army was south of Market Bosworth in Leicestershire, some four miles from the King's force. Henry's army by this time numbered between 5000 and 7000 men, while Richard's army was around 8000-12000 strong.

The battle began as a race between the two armies to secure the high ground midway between them, a feature known as Ambion Hill. Richard won the race, and deployed the vanguard of his army, under the Duke of Norfolk, across the crest.

There was not room for the whole of his force to deploy along the ridge, however, and his center and rearguard had to be positioned behind. As Henry's army approached the hill he discovered that his path was blocked by a marsh, and had to detour to the north to avoid it; this manoeuver exposed his army to numbers, seemed to give Richard the advantage. At this moment, however, an additional factor arose in the shape of some 8000 men in two armies commanded by two of the most powerful nobles in England, Lord Thomas Stanley and his brother Sir William Stanley. They did not join one side or the other, but positioned themselves on either side of the field, so that the four armies made up the four sides of a square. The ambiguity of this positioning was clear; if the Stanleys joined Richard his victory was certain; if they joined Henry the Tudor cause would triumph. The situation was further complicated by the fact that Richard was holding William Stanley's son as a hostage against his loyalty. If he chose his moment carefully, he might save his son's life and be on the winning side, so when Henry sent an invitation to him to join his cause his answer was ambiguous and he made no move. Thus both commanders began the battle in a state of doubt.
After an initial bout of cannon fire, the two armies of Richard and Henry advanced upon each other and met halfway down the hill. The two vanguards became embroiled in a savage melee for something over an hour, with the entire center of each army gradually becoming drawn in, in support of the vanguards. It was during this period of struggle that the Stanleys began to move towards the fighting, and that the course of the battle becomes much less clear. It appears that the two sides had somewhat disengaged, with the strain of fighting uphill beginning to tell on Henry's force, and that as the armies drew apart Richard sighted Henry on rising ground west of his main force and decided a direct engagement with his rival could decide the battle before the Stanleys made a move which would perhaps take victory out of his reach.

He requested aid from his ally the Earl of Northumberland, but the latter replied that he must remain where he was and watch Stanley, leaving Richard with the knights and esquires of his own force to lead his reserve in an attack on Henry. It was as he was charging across the front of William Stanley's force in an effort to reach Henry that Stanley made his decision and advanced against the King's party. In the ensuing fighting it seems that Richard succeeded in getting close to Henry before he and his force were overwhelmed and he was unhorsed and killed.

The death of King Richard III meant the end of the battle as decisively as Stanley's decision to side with Henry. With the King dead, his army had no cause to fight for, and the battle ended in a rout as the King's men broke and fled at the news of his fall. Henry Tudor was crowned as King Henry VII on the field of battle, beginning a new dynastic age.

A clearer case of a battle the outcome of which had extensive political consequences could hardly be imagined. One man was king at the beginning of the day, another was king at the end of it, and the cause of the former was entirely lost. With the exception of some quickly extirpated rebellions no civil strife followed Henry's victory, and his dynasty was rapidly, and firmly, established.

But what if the battle had gone to Richard? If Henry had been defeated and fled, much depends on whether he could have re-assembled his faction and mounted a further invasion, perhaps when Richard's position was once more a weak one. Certainly Richard had no certainty of his place on the throne being secure, even if he had won. He had no heir and no queen, and even if he had re-married and had another son it is likely that he would not have come of age by the time he succeeded, leaving England with the possibility of a child monarch, a protectorate, and further internal strife. If any of the Lancastrian tendency had survived the battle and were able to mount a challenge, perhaps with foreign support, they would probably have found plenty of opportunity to do so.

The other point is that Richard would have presided over a divided court. Victory in battle would have removed the challenge posed by many of those who opposed him among the nobility, but many would have remained in England and overseas, and with every difficulty the King…