On 20 May 1217, royalist forces relieved Lincoln Castle and defeated a French-backed rebel army during the First Barons’ War.

The battle came two years after Magna Carta, but England was still unsettled. King John had died in 1216, leaving his nine-year-old son, Henry III, as king. That changed the war. The quarrel over John’s rule had become a fight over whether a child king, governed by regents, could hold the kingdom against rebel barons and Prince Louis of France.

Lincoln gave that wider contest a fixed point. The city was in rebel hands, but Lincoln Castle still held for the royalists. Its constable, Nicholaa de la Haye, had remained loyal to the king’s side. As long as the castle held out, the royalists had a foothold inside a hostile city.

William Marshal led the response. By then, he was an old man. Still, he was also the protector of Henry III’s government and one of the few figures with the authority to hold the royalist cause together. His task was practical: relieve the castle, defeat the rebel force and prove that the king’s side could still act.

The royalist army reached Lincoln and attacked the rebel position. The fighting broke the siege and ended with a clear royalist victory. Thomas, Count of Perche, one of the French commanders, was killed. Many rebel leaders were captured. The city was then looted by the royalist army, giving the battle its later nickname, the ‘Lincoln Fair’.

The victory did not end the war. Prince Louis still had support, and the final settlement came only after more pressure and negotiation. But Lincoln mattered. The rebel and French-backed army had been beaten, and Henry III’s government looked less fragile than it had before the battle.

That is the force of the date. Magna Carta had not made peace by itself. After John’s death, the future of the kingdom was still being decided by loyalty, castles, armed force and political calculation. At Lincoln, the child king’s cause held.