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Prologue:

Prince Louis caught wind that Rochester Castle had fallen to King John and that he'd hung the majority of the rebel defenders*, however he pressed on with the Siege of Windsor Castle which finally yielded before the new year of 1217**. From here Louis chose to retake Rochester Castle and also to finally take Dover Castle, while he did this Robert Fitzwalter took his army North to take back the lands recently lost to King John I.

At around the same time King John's eldest son Prince Henry died of dysentery***, when King John heard of this tragedy his moral utterly collapsed however he continued to fight on. Bad news continued to flood his way though as by March of 1217 the important port of Dover had finally fallen to Prince Louis' troops and Fitzwalter had managed to retake the lands that had been taken by Royal forces some months beforehand.

With the capture of Dover Prince Louis was able to bring in reinforcements from France and the continent while King John was doing the same in Kings Lynn to the North. However many of Louis' reinforcements were veterans of the campaigns against English Normandy thus were already battle-hardened. Not long after this Prince Louis took his army West to attack the Royalist Corfe Castle in Dover, while Robert continued his march towards Lincoln.

The army of King John moved southwards towards Kent in order to recapture Canterbury Castle and then move onto Rochester Castle yet again. The armies of John and Robert missed each other by a few miles as they marched and yet neither knew of the others existence for the time being.

After having its foundations undermined and the North Gate collapsing Corfe Castle surrendered completely to Prince Louis and his troops after a month-long siege. King John's campaign seemed to be falling part around him with practically all the important ports and castles in the South being under the control of Prince Louis, however even during these dire situations John refused to give in as he still had his Northern power-base.

Robert Fitzwalter began the Siege of Lincoln yet again as he hoped to destroy the power-base that King John had left, again the Earl of Pembroke was defending Lincoln however now the rebel force had one of the main commanders and was a much larger force. In the proceeding Battle of Lincoln the Earl of Pembroke and a large number of his forces were cut down by the Barons' forces. It's been noted that this was the true turning point in the war.

Once Lincoln had fallen to Fitzwalter the Scottish troops that had been slowly progressing in the North began to move quickly, the army led by Alexander II of Scotland began to siege Alnwick Castle. Alongside this by July Louis essentially controlled all of the Southern coast of England, at which point he personally took his army eastwards to relieve the siege on Rochester being conducted by King John I.

In August the two forces met, the rebels led by Prince Louis against the Royal forces led by King John I. The largest battle to have occurred since Hastings on English soil occurred and after around 7 hours of fighting Prince Louis came out as the winner, while King John lay in the dust having been killed during the battle.

Louis didn't wait at this point, he rode into London and before the end of August on the 27th he was crowned King Louis I of England with almost all of the Barons' supporting his rise to become English monarch. At the same time Fitzwalter whom hadn't been informed had begun his siege of York, attempting to take Clifford's Tower which had previously been John's stronghold.

*IOTL King John didn't do this, it's a minor POD however it helps to create an overwhelming hatred of him and turn some people whom were previously neutral to him.
**IOTL Windsor Castle didn't fall.
***IOTL John caught dysentery, however now it's his 9 year old son whom does thus weakening John.
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