Bishop Beck raised a large army in Northumberland ready to march on Wallace. Bruce, who was in the area, had been persuaded by Edward's lies that Wallace, a mere commoner, in fact sought the crown of Scotland for himself and not for its rightful king. He therefore decided to join Beck and win the crown of Scotland for himself, regarding Wallace as a rebel and usurper. So the man for whom Wallace sought the crown in fact was deceived into believing that loyal hero, his best friend, to be his enemy. They marched north and set up camp at Norham. Meanwhile Corspatrick marched with an army to Dunbar where he laid siege to the castle. Ships were sent from Newcastle to blockade Dunbar harbour, preventing supply ships from entering or leaving. Wallace had managed to raise an army of some five thousand men and now set out for Dunbar to help Seton and raise the siege. At Yester he was met by Hay and his fifty well-armed men and informed of Corspatrick's strength. Wallace resolved to press on to the attack, not knowing of Beck's army lying in wait at Norham.

Corspatrick now laid a trap for Wallace. He decided to meet Wallace with all his strength outside Dunbar at a place called Spot Muir. He sent word to Beck and Bruce of his intention and told them to approach from a direction which would take Wallace unawares from the side, if not indeed in time to ambush him. When Seton saw the Corspatrick army in the field he sallied out with some of his best men, leaving the rest to defend the castle, intent on helping Wallace. Some of Wallace's comrades, when they saw the strength of Corspatrick's army advised against fighting a pitched battle against such odds. Not knowing of Beck's hidden army, Wallace refused the advice saying one of his men was worth four of the others, and charged. The ensuing battle was ferocious, the loyal Scots fighting like tigers at bay. Corspatrlck himself was a warrior of the best and but for his own example his men would have broken before they did. As it was, after some heavy fighting and heavier losses, Corspatrick's men began to give way and flee, but at that moment Beck's army came on to reinforce them.

Wallace was utterly shocked and pained to see the Bruce's standard flown against his own people. It was a blow no other could have struck against him. He thought of ordering his men off the field, then realised it was almost impossible to do anything now but fight on. That day, Bruce slew many of his own people, as did the traitor Corspatrick and Bishop Beck. Wallace sought to get through to the Earl himself but was prevented by one Maitland, whom he slew. Corspatrick's sword caught Wallace on the thigh, wounding him, but before Wallace could get back at him the press of men separated them. Wallace soon found himself isolated from his own men and surrounded by foes. His mighty sword kept a clearance round him, but Corspatrick ordered his spearmen to bring him down, but the great sword shore the head off any spear that came near him. The Scots soon noticed Wallace was isolated and sought to aid him, Graham, Lauder, Lyall and Hay chief among them, with Lundy, Boyd and Seton. Beck was knocked from his horse in this melee, and before he got to his feet Wallace had been rescued from his tight corner. Now hopelessly outnumbered, Wallace ordered a retreat, and they escaped to the forest where Corspatrick and the others would find it difficult to follow. Beck and his army camped that night in the Lammermuirs, Corspatrick fuming and fretting at his bad luck. Beck feared that Wallace, whose worth against overwhelming odds had been proven that day, would lose no time in gathering reinforcements and return for revenge.