Since 1332 the disinherited - Scottish nobles who had refused fealty to Robert the Bruce and lost their lands - had been trying to install Edward Balliol on the throne of Scotland in place of the young David II. Two crushing victories at Dupplin Moor (1332) and Halidon Hill (1333) had nearly annihilated the Scottish governing class, but Balliol could not hold what he had won. Twice he was crowned. Twice he was driven out. In 1335 Edward III decided to settle the question himself. He gathered over thirteen thousand men at Newcastle and divided them: he would lead one wing through Carlisle, Balliol the other through Berwick, with naval support up both coasts. The objective was not castles but the Scottish army. Like biblical locusts, Balliol's column stripped Lothian. Newbattle Abbey and Manuel Nunnery burned despite letters of royal protection. Inchcolm Abbey was sacked from the sea. By the end of July the two arms met at Glasgow. They had caught nothing of substance.

The Scots had refused to give battle. John Randolph, 3rd Earl of Moray and Guardian of Scotland, kept his army small and mobile, harassing English supply columns from the woods. His chance came when Guy of Namur made his rash march from Berwick. Guy was a Fleming, cousin of Queen Philippa of England, and chivalric ambition appears to have been his real motive - he was not even properly committed to Edward's war. Moray's scouts likely watched him from the moment he crossed the Tweed. As Guy approached Edinburgh from the south he was ambushed on the Borough Muir, the common grazing ground that gave the battle its name. He fought his way through. Then Sir William Douglas came down from the Pentland Hills with more Scots, and Guy's situation collapsed.

With nowhere else to run, Guy made for Edinburgh. His retinue entered the town by the Friars' Wynd, fought up St Mary Wynd, and climbed to the castle rock - then a smoking ruin. Edinburgh Castle had been deliberately demolished in 1314 by Sir Thomas Randolph, after Bruce recaptured it from the English, to prevent any future English garrison from using it. The defences were gaps; the gaps had to be filled. They killed their exhausted horses and built a wall of their bodies. The chronicler Walter Bower described what followed: 'And thus, surrounded and besieged by the Scots throughout the whole of that night, they passed it continuously without sleep, hungry, cold, thirsty and weary. Tired out and distressed in this way, and with no hope of any help, they in the morning of the next day surrendered themselves to the Scots.'

Moray could have killed Guy and ransomed his survivors. Instead he negotiated a chivalric capitulation: Guy and his men were allowed to leave Scotland on swearing never again to take arms against King David. There was a political reason too. Guy was a subject of Philip VI of France, and France was Scotland's vital ally. The Guardian even rode personally with Guy toward the border. The decision cost him everything. An English force from Jedburgh under William Pressen ambushed Moray on the way back. Sir William Douglas escaped; his brother James was killed; Moray himself was captured and would spend the next five years in English jails. Guy returned to Berwick and sailed with Queen Philippa to meet Edward at Perth, where he was politely received and then politely sent home. His chivalric adventure had cost him his reputation. English chroniclers censured him for entering enemy territory with too small a following. He never came back.