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War with the barons

The rebels made the first move in the war, seizing the strategic Rochester Castle, owned by Langton but left almost unguarded by the archbishop. John was well prepared for a conflict. He had stockpiled money to pay for mercenaries and ensured the support of the powerful marcher lords with their own feudal forces, such as William Marshal and Ranulf de Blondeville, 6th Earl of Chester. The rebels lacked the engineering expertise or heavy equipment necessary to assault the network of royal castles that cut off the northern rebel barons from those in the south. John's strategy was to isolate the rebel barons in London, protect his own supply lines to his key source of mercenaries in Flanders, prevent the French from landing in the south-east, and then win the war through slow attrition. John put off dealing with the badly deteriorating situation in North Wales, where Prince Llywelyn was leading a rebellion against the 1211 settlement.

John's campaign started well. In November John retook Rochester Castle from rebel baron William d'Aubigny in a sophisticated assault. One chronicler had not seen "a siege so hard pressed or so strongly resisted", whilst historian Reginald Brown describes it as "one of the greatest operations in England up to that time". Having regained the south-east John split his forces, sending William Longespée to retake the north side of London and East Anglia, whilst John himself headed north via Nottingham to attack the estates of the northern barons. Both operations were successful and the majority of the remaining rebels were pinned down in London. In January 1216 John marched against Alexander II of Scotland, who had allied himself with the rebel cause. John took back Alexander's possessions in northern England in a rapid campaign and pushed up towards Edinburgh over a ten-day period.

The rebel barons responded by inviting Prince Louis of France to lead them: Louis had a claim to the English throne by virtue of his marriage to Blanche of Castile, a granddaughter of Henry II. Philip may have provided his son with private support but refused to openly support Louis, who was excommunicated by Innocent for taking part in the war against John. Louis' planned arrival in England presented a significant problem for John, as Louis would bring with him naval vessels and siege engines essential to the rebel cause. Once John contained Alexander in Scotland, he marched south to deal with the challenge of the coming invasion.

Louis intended to land in the south of England in May 1216, and John assembled a naval force to intercept him. Unfortunately for John, his fleet was dispersed by bad storms and Louis landed unopposed in Kent. John hesitated and decided not to attack Louis immediately, either due to the risks of open battle or over concerns about the loyalty of his own men. Louis and the rebel barons advanced west and John retreated, spending the summer reorganising his defences across the rest of the kingdom. John saw several of his military household desert to the rebels, including his half-brother, William Longespée. By the end of the summer the rebels had regained the south-east of England and parts of the north.