The Truce of Leulinghem was unravelling, and along the Anglo-Scottish frontier both sides had returned to the old habit of raiding. In June, the English-aligned George Dunbar caught a Scots raiding party at Nesbit Moor and gave no quarter. Archibald Douglas, fourth Earl of Douglas and arguably the most powerful military lord in Scotland, took Nesbit as the pretext for a punitive expedition. He gathered ten thousand men, marched south with Murdoch of Fife at his side, and laid waste to Northumberland as far as Newcastle. The chronicles of plunder were standard for the border. The army on the way home, fat with loot and tired, was the real prize. Henry Percy and his son Hotspur understood this perfectly. Persuaded by George Dunbar, they lay in wait at Wooler.

When the English rushed Douglas's camp at Milfield, Scottish sentries gave enough warning. The army withdrew to higher ground on Homildon Hill, the modern Humbleton, and formed into schiltrons - the dense pike-and-spear hedgehog formations that had served Scotland for generations. Douglas had not learned from his great-uncle, who had died at Halidon Hill seventy years earlier facing the same English tactic on a similar slope. A schiltron is a magnificent defensive formation against cavalry. It is a stationary target against archery. The English longbowmen drew up below and began to shoot. The bodkin points punched through cloth, through mail, through faces. The schiltrons began to break apart from the inside as men fell or flinched or tried to push back from the front rank where the arrows landed thickest.

Sir John Swinton of that Ilk could see what was happening and what would happen next. He gathered a hundred men and made the only decision left to him. They drew their swords and ran downhill into the English archers and men-at-arms, and they were cut down to a man. The chronicles preserve his words: better to die in the mellay than be shot down like deer. Douglas, watching from the main body, hesitated to advance. When he finally signalled the assault, his army was already half-destroyed. The Scots who reached the English line met fresh, unbloodied men-at-arms and were routed in minutes. Douglas himself was wounded five times. He lost an eye. The armour he had supposedly spent three years commissioning could not stop a longbow arrow at close range, because nothing could. He was captured along with George Douglas of Angus, Thomas Dunbar of Moray, Murdoch of Fife, and Henry Sinclair of Orkney. The cream of the Scottish nobility went into English captivity.

Henry IV refused to let the Percys ransom their prisoners. Ransoms were how medieval war paid for itself, and the Percys had captured a king's worth of Scottish nobility. Henry's logic was straightforward: he did not want experienced Scottish commanders returning home to fight him on a future battlefield. The logic was sound. The political consequences were catastrophic. The Percys had grievances with the Crown already, and this one became unbearable. In 1403 they allied themselves with Owain Glyndwr in Wales and rose in open rebellion. Hotspur freed his Scottish prisoners on the way to fight his king, and many of them, including Douglas himself, joined him. Douglas fought again at the Battle of Shrewsbury and was wounded again. Hotspur died there with an arrow through the face. Shakespeare wove all of this into Henry IV, Part 1, beginning his play with the news from Holmedon's plains arriving at the king's court.

Humbleton Hill rises above the small Northumbrian village of Wooler, inside the Northumberland National Park. The summit holds an Iron Age hillfort built fifteen hundred years before Douglas and Hotspur fought below it - the medieval combatants would have looked up at ruins already ancient in their time. The Battle Stone, traditionally believed to mark the battle, turns out to be a Bronze Age standing stone older still. Layers of war stacked on a single hillside, each generation borrowing the last one's stones. Stand in the right place on a clear day and you can still see the marshy hollow the schiltrons defended, and the rising ground from which the English arrows came down.