How did Henry Tudor become the main challenger to Richard III?

Henry Tudor became the primary challenger to Richard III largely due to the political instability following Richard’s rise to power in 1483. Richard’s usurpation split the Yorkist establishment, alienating many county elites and former supporters who grew disillusioned by rumors that he had murdered his nephews.123

Following a failed rebellion in 1483, many of these disaffected Yorkists fled to Brittany, where they joined Lancastrian exiles and formally recognized Henry Tudor as king on Christmas Day. Henry, the sole surviving male with a claim to the house of Lancaster, became the focal point for this diverse coalition of opponents. To solidify this alliance, Henry promised to marry Elizabeth of York, the daughter of Edward IV, which provided a bridge between the warring factions.123

Supported by French and Scottish mercenaries, Henry landed at Milford Haven in 1485. His challenge culminated at the Battle of Bosworth Field, where the desertion of key figures like Lord Stanley proved decisive in defeating Richard III.1423