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Continental policy

During the remainder of his reign, John focused on trying to retake Normandy. The available evidence suggests that he did not regard the loss of the Duchy as a permanent shift in Capetian power. Strategically, John faced several challenges: England itself had to be secured against possible French invasion, the sea-routes to Bordeaux needed to be secured following the loss of the land route to Aquitaine, and his remaining possessions in Aquitaine needed to be secured following the death of his mother, Eleanor, in April 1204. John's preferred plan was to use Poitou as a base of operations, advance up the Loire Valley to threaten Paris, pin down the French forces and break Philip's internal lines of communication before landing a maritime force in the Duchy itself. Ideally, this plan would benefit from the opening of a second front on Philip's eastern frontiers with Flanders and Boulogne—effectively a re-creation of Richard's old strategy of applying pressure from Germany. All of this would require a great deal of money and soldiers.

John spent much of 1205 securing England against a potential French invasion. As an emergency measure, he recreated a version of Henry II's Assize of Arms of 1181, with each shire creating a structure to mobilise local levies. When the threat of invasion faded, John formed a large military force in England intended for Poitou, and a large fleet with soldiers under his own command intended for Normandy. To achieve this, John reformed the English feudal contribution to his campaigns, creating a more flexible system under which only one knight in ten would actually be mobilised, but would be financially supported by the other nine; knights would serve for an indefinite period. John built up a strong team of engineers for siege warfare and a substantial force of professional crossbowmen. The King was supported by a team of leading barons with military expertise, including William Longespée, 3rd Earl of Salisbury, William the Marshal, Roger de Lacy and, until he fell from favour, the marcher lord William de Braose.

John had already begun to improve his Channel forces before the loss of Normandy and he rapidly built up further maritime capabilities after its collapse. Most of these ships were placed along the Cinque Ports, but Portsmouth was also enlarged. By the end of 1204 he had around 50 large galleys available; another 54 vessels were built between 1209 and 1212. William of Wrotham was appointed "keeper of the galleys", effectively John's chief admiral. Wrotham was responsible for fusing John's galleys, the ships of the Cinque Ports and pressed merchant vessels into a single operational fleet. John adopted recent improvements in ship design, including new large transport ships called buisses and removable forecastles for use in combat.

Baronial unrest in England prevented the departure of the planned 1205 expedition, and only a smaller force under William Longespée deployed to Poitou. In 1206 John departed for Poitou himself, but was forced to divert south to counter a threat to Gascony from Alfonso VIII of Castile. After a successful campaign against Alfonso, John headed north again, taking the city of Angers. Philip moved south to meet John; the year's campaigning ended in stalemate and a two-year truce was made between the two rulers.

During the truce of 1206–1208, John focused on building up his financial and military resources in preparation for another attempt to recapture Normandy. John used some of this money to pay for new alliances on Philip's eastern frontiers, where the growth in Capetian power was beginning to concern France's neighbours. By 1212 John had successfully concluded alliances with his nephew Otto IV, a contender for the crown of Holy Roman Emperor in Germany, as well as with the counts Renaud of Boulogne and Ferdinand of Flanders. The invasion plans for 1212 were postponed because of fresh English baronial unrest about service in Poitou. Philip seized the initiative in 1213, sending his elder son, Louis, to invade Flanders with the intention of next launching an invasion of England. John was forced to postpone his own invasion plans to counter this threat. He launched his new fleet to attack the French at the harbour of Damme. The attack was a success, destroying Philip's vessels and any chances of an invasion of England that year. John hoped to exploit this advantage by invading himself late in 1213, but baronial discontent again delayed his invasion plans until early 1214, in what was his final Continental campaign.