Earldom of Northumberland returned to the Percy Family

Earldom of Northumberland returned to the Percy Family is in 1469-1479 Wars of the Roses: Warwick Rebels.

On 27 Mar 1470 George Neville 1st Duke Bedford (age 9) was created 1st Duke Bedford by King Edward IV of England (age 27) in preparation for his marriage to Elizabeth York Queen Consort England (age 4) which didn't, in the end, take place. He, George, was nephew to Richard "Kingmaker" Neville Earl Warwick, 6th Earl Salisbury (age 41) whose defection to the Lancastrian side may have caused the King to change his mind about his daughter's marriage.

Warkworth's Chronicle 1470. 27 Mar 1470. And in the same x. yere aforeseide, a lytelle before Michaelmesse, the Duke of Clarence and the Erle of Warwyke (age 41) londede in the west countre, and gadered there a grete peple. The Lorde Markes Montagu (age 39) hade gaderyd vi. Ml men, by Kynge Edwardes commysyone and commaundement, to the entente to have recistede the seide Duke of Clarence, and the Erle of Warwyke (age 41). Nevere the lattere, the seide Markes Montagu (age 39) hatyde the Kynge, and purposede to have taken hym; and whenne he was withein a myle of Kynge Edwarde, he declarede to the peple that was there gaderede with hym, how Kynge Edwarde hade fyrst yevyne to hym the erledome of Northumberlonde, and how he toke it from hym and gaff it Herry Percy, whos fadere was slayne at Yorke felde; and how of late tyme hade he made hym Markes of Montagu (age 39), and yaff a pye's neste to mayntene his astate withe: wherefor he yaff knoleage to his peple that he wulde holde withe the Erle of Warwyke (age 41), his brothere, and take Kynge Edwarde if he myght, and alle tho that wolde holde with hym.

On 27 Mar 1470 John Neville 1st Marquess Montagu (age 39) revoked his title of Earl of Northumberland so that the Earldom could be given to Henry Percy 4th Earl of Northumberland (age 21) of the Percy family who were its traditional holders. A deeply unpopular move with the Neville family who had been longstanding enemies of the Percies in the North. He was created 1st Marquess Montagu instead. Possibly a consequence of the defection of Warwick the Kingmaker (age 41) to the Lancastrian cause as evidenced by the Welles Rebellion earlier in the year.