A skilled knight, Hereford was Edward’s brother-in-law – but had not always supported him. In fact, Hereford had been implicated in the death of Edward's favourite Piers Gaveston in 1312. As Constable of England he had rights of command in the army and had experienced the realities of fighting as well as the chivalry of the jousting field.
He had been given Bruce’s own castle of Lochmaben by Edward I and so must have felt a personal edge to the conflict.
Hereford quarrelled with Gloucester as to who had the right to lead the vanguard, the most prestigious position in the army. The king decided that the duty should be shared, but this just caused confusion in the English army.
On 23 June as the cavalry approached the Scots, Hereford witnessed the slaughter of his nephew, Henry de Bohun, by Bruce himself. His cavalry could not break through the Scots in the New Park. Next day Hereford’s men had another disastrous encounter with the Scots, with no room to manoeuvre and being powerless against the spearmen.
Hereford fled to English-controlled Bothwell Castle – but found himself trapped when the keeper of the castle switched sides and handed him over to Edward Bruce, King Robert's brother. He was a valuable hostage, and was exchanged for a ransom of important prisoners, including Bruce’s wife Elizabeth, daughter Marjory, sisters Christian and Mary, and blind Bishop Robert Wishart of Glasgow.