The Act of Accord (25 October 1460) was a political agreement that was meant to solve the political tension that had caused war in England in the 1450s by acknowledging Richard, duke of York, as the heir to Henry VI, but that instead helped reinvigorate the Lancastrian cause.
The first phase of the Wars of the Roses, in 1455, had been very short, involving only the first battle of St. Albans (22 May 1455) and its aftermath. At this stage Richard, duke of York had been claimed that he was a loyal supporter of Henry VI, and only wanted to reform his government. His particular enemy, Edmund Beaufort, second duke of Somerset, had been killed in the battle, but York hadn't enjoyed power for long. Henry's wife Margaret of Anjou led the Lancastrian fight-back and York and his supporters were driven out of political power. In 1458 Henry made an attempt to end the tension with his 'Loveday' agreement of 24 March 1458, but this failed and in 1459 the second phase of the Wars of the Roses began.
In 1459 the Lancastrians were triumphant. Although the Yorkists were able to unite their three main armies (under York, Richard Neville, earl of Salisbury and his son Richard Neville, earl of Warwick), they were still outnumbered by the Lancastrians. The two sides ended up facing each other at Ludford Bridge, just to the south of Ludlow (12-13 October 1459). Their most professional troops were members of the garrison of Calais, but when they discovered that Henry VI was present in person in the Royal army, they changed sides. The Yorkist leaders realised that they couldn't win and on the night of 12-13 October they abandoned their army and fled into exile. York went to Ireland while the Nevilles, along with York's son Edward, earl of March, reached Calais.
In 1460 the Yorkists fought back. In June 1460 Warwick, Salisbury and March landed at Sandwich. They then marched to London, where Salisbury was left to besiege the Tower. Warwick led the Yorkist army north, and at Northampton (10 July 1460) they defeated Henry's army and captured the king. Once again Warwick and his men had claimed to be loyal supporters of Henry, and he was escorted back to London with full honours.
For the next few months Warwick ruled while the country waited for York to return. He crossed over from Ireland in September and was visited by Warwick at Shrewsbury. Warwick then returned to London, where Parliament was sitting, while York made a slow progress towards London.
On 10 October 1460 York made an attempt to claim the throne. He entered the Parliament chamber and placed his hand on the King's throne. He was clearly expecting to be acclaimed by the Peers, presumably on the grounds that they were sick of Henry's incompetent rule. Instead he was greeted with silence before Thomas Bourchier, Archbishop of Canterbury, asked if he wanted to see the king. York had badly misjudged the situation - Henry's favourites and his Queen were unpopular in some circles, but Henry himself was still the legitimate king, and would have a reasonable amount of support until his death. It isn't clear if Warwick, Salisbury and March were involved in this plot - they are claimed to have been furious with York's attempt to seize the throne, but this may have been false anger put on after the effort failed or anger with his botched attempt. It is also possible that Warwick was involved in the scheme but that his father Salisbury was not.
On 16 October York formally laid out his case in front of Parliament. His basic claim was that the Lancastrians were usurpers, and that his claim to the throne was more direct. The Lancastrian kings were descended from John of Gaunt, the third son of Edward III. York was descended from Edward III in two lines - on the paternal side the link was to Edmund, first duke of York, Edward's fourth son, but his claim came on the maternal side. His mother Anne Mortimer was the daughter of Roger Mortimer, earl of March, the son of Edmund Mortimer, earl of March and Philippa, daughter of Lionel, duke of Clarence, Edward III's second son. The last male heir to the Mortimer claim, Edmund Mortimer, earl of March, died in 1425, and his claim passed to York.
Parliament rejected York's claim on the grounds that they had all taken repeated oaths of loyalty to the Lancastrian dynasty and to Henry VI (as had York himself) and that the Lancastrian claim was firmly established by statute).
Over the next few days frantic negotiations were carried out and a compromise was finally agreed. Henry VI was to remain on the throne for the rest of his life, but his son Prince Edward was disinherited. Instead the succession passed Richard of York and his heirs (York was actually ten years older than Henry VI so if the act had been accepted peacefully his son Edward was most likely to have benefited from this). York and his sons were granted an income of 10,000 marks from the earldom of Chester, normally held by the Prince of Wales. York was given control of the government, with similar powers as during his protectorates.
The Act of Accord was accepted by Henry VI and York on 25 October and passed by Parliament. On 31 October the Lords swore to accept York as heir to the throne, while York swore to accept Henry as king.
The big weakness with the Act of Accord was the fate of the young Prince Edward. While Henry had lost much support because of the weakness of his government, his infant son wasn't affected by this and became a great rallying point for Lancastrians who might not otherwise have fought. The Act of Accord also broke one of the guiding principles of the Medieval aristocracy - the right of a son to inherit their father's lands and titles. Richard II had made the same mistake when he disinherited Henry Bolingbroke, triggering his invasion and usurpation as Henry IV.
In the aftermath of the Act of Accord a series of Lancastrian revolts broke out around the country, led by Queen Margaret, who had escaped to Wales after Northampton. The strongest threat emerged in the north, and late in 1460 York headed north to deal with it. For once he had moved too quickly and was outnumbered. On 30 December 1460 York attacked a larger Lancastrian army at Wakefield, and was killed in the battle. Salisbury killed on the following day and leadership of the Yorkist cause passed to Edward, earl of March and Richard Neville, earl of Warwick. York had thus only benefited from the terms of the Act of Accord for two months before his death and it would be his son who came to the throne as Edward IV.