(1) A quote from The Brothers York: An English Tragedy:

“Back in I483, Northumberland’s military might had been instrumental in Richard’s rise to power. But, with Richard refusing to hand Northumberland control of the northeast, all the earl’s resentments about Richard – of how, in the I470s, Richard had muscled into the region he regarded as his birthright – had come flooding back. Now, Northumberland stayed where he was. Later, some tried to explain away his inaction, stating that the earl had been physically unable to advance through the boggy ground in front of him. But Northumberland had a history of ‘sitting still’. Back in I471, as Edward IV advanced south to reclaim his throne, Northumberland had done nothing: a non-intervention that, as one of Edward’s men had remarked, was in fact a ‘notable service’. Whatever the case, with both Stanley and Northumberland refusing to budge, Richard had to act.”

(1a) Here’s Grok on Richard’s isolation at Bosworth:

Henry Percy, Earl of Northumberland, commanded a large contingent on Richard’s side but failed to engage or advance his troops meaningfully (some sources describe this as passive betrayal or deliberate inaction due to regional rivalries or caution).

(2) King Richard III DNA Reveal Was So Shocking They Tried To Hide It, Now In 2025 The Truth Comes Out (YouTube)