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The Battle of Barnet, which took place on April 14, 1471, was a decisive battle of the Wars of the Roses (wikipedia.org), near the town of Barnet, 10 miles north of London.
The main protagonists were King Edward IV of England and Richard Neville, Earl of Warwick, former friends and allies who had fallen out as a result of Edward's tendency to favour the relatives of his queen, Elizabeth Woodville. In October of the previous year, Warwick "the Kingmaker" had driven Edward out of the country, replacing his Lancastrian predecessor, King Henry VI of England, on the throne. Warwick then made the mistake of agreeing to assist King Louis XI of France in his conflict with the Duke of Burgundy. This prompted the Burgundians to offer military aid to Edward, who returned to England on March 14, 1471. The two armies were evenly matched in numbers, but Warwick was expecting support from his son-in-law, George, Duke of Clarence, who happened to be Edward's brother and hurried to make his peace with the latter.
Edward marched to London while Warwick remained in Coventry where he had been raising troops. Having taken back his capital, Edward then moved to meet Warwick at Barnet.
On the Lancastrian side, the Earl of Oxford commanded the right flank, the Marquess of Montagu the center, and the Duke of Exeter the left. These were arrayed offset from the road to Barnet, with the left flank on the road. The Earl of Warwick was with the reserves.
The Yorkists were set-up centered on the road, with Edward commanding the center, his brother Richard (later Richard III) on the right, and Lord Hastings the left.
Oxford's troops, helped by their initial flanking position made an early impact on the Yorkist left, but this was compensated for by foggy conditions which made it difficult for the Dukes of Somerset and Exeter to push home their advantage. The two sides ended up so the battle lines were parallel to the highway rather than at right angles to it. Some of the Lancastrian troops mistook one another for the enemy in the ensuing confusion. As the mist cleared and Warwick recognised defeat, he was cut down while trying to reach his horse. His younger brother, the Marquess of Montagu, was also killed, probably by one of Oxford's troops when the cry of treachery went up.
The same day as the battle Queen Margaret landed at Weymouth and began gathering troops from Wales and the Welsh Marches. Had she been able to join forces with Warwick matters might have ended very differently. Instead, Edward's victory enabled him to consolidate and prepare himself for the final confrontation against the Lancastrian royal family - the Battle of Tewkesbury.
Battle of Barnet, by John Warkworth Chronicle. (London: 1839)
Wars of the Roses (Wikipedia)