During the winter the barons had kept up their negotiations with Louis, and an agreement had finally been made. They had pledged themselves to do homage to Louis and accept him as king, and had sent to France twenty-four hostages "of the noblest of the land" in pledge of their fidelity. Louis in return sent over small bodies of men to their aid and promised himself to follow in person in the spring. To this step the barons were indeed driven, unless they were prepared to submit, because of the strength the king had gained since the signing of the charter and their own comparative weakness. Why this change had taken place so soon after the barons had been all-powerful cannot now be fully explained, but so far as we can see the opinion of a contemporary that they would have been overcome but for the aid of the French is correct. Against the invasion of Louis, John had two lines of defence, the pope and the fleet. Innocent, who had once favoured a transfer of the English crown to Louis, must now oppose it. When he learned how far preparations for the expedition had gone, he sent a legate, Cardinal Gualo, to France to forbid any further step. Gualo was received by Philip and his son at Melun on April 25. There before the king and the court the case was argued between the cardinal and a knight representing Louis, as if it were a suit at law to be decided in the ordinary way. Louis's case was skilfully constructed to deprive the legate of his ground of interference, but his assertions were falsehoods or misrepresentations. John had been condemned to death for the murder of Arthur - the first occasion on which we hear of this - and afterwards rejected by the barons of England for his many crimes, and they were making war on him to expel him from the kingdom. John had surrendered the kingdom to the pope without the consent of the barons, and if he could not legally do this, he could by the attempt create a vacancy, which the barons had filled by the choice of Louis. The legate, apparently unable to meet these unexpected arguments, asserted that John was a crusader and therefore under the protection of the apostolic see. For Louis it was answered that John had been making war on him long before he took the cross and had continued to do so since, so that Louis had a right to go on with the war. The legate had no answer to this, though it was false, but he prohibited Louis from going and his father from allowing him to go. Louis, denying the right of his father to interfere with his claims in a land not subject to the king of France, and sending an embassy to argue his case before the pope, went on with his preparations. Philip Augustus carefully avoided anything that would bring him into open conflict with Innocent and threw the whole responsibility on his son.
Louis landed in England in the Isle of Thanet on May 21. John had collected a large and strong fleet to prevent his crossing, but a storm just at the moment had dispersed it and left the enemy a clear passage. John, then at Canterbury, first thought to attack the French with his land forces, but fearing that his hired troops would be less loyal to a mere paymaster than to the heir and representative of their suzerain in France, he fell back and left the way open for Louis's advance to London. Soon after landing, Louis sent forward a letter to the Abbot of St. Augustine's in Canterbury, who, he feared, was about to excommunicate him. In this letter which was possibly intended also for general circulation, he repeated the arguments used against the legate with some additional points of the same sort, and explained the hereditary claim of his wife and his own right by the choice of the barons. The document is a peculiar mixture of fact and falsehood, but it was well calculated to impose on persons to whom the minor details of history would certainly be unknown. Rochester castle fell into the hands of the French with no real resistance; and on June 2, Louis was welcomed in London with great rejoicing, and at once received the homage of the barons and of the mayor. Louis's arrival seemed to turn the tide for the moment against the king. He retreated into the west, while the barons took the field once more, and with the French gained many successes in the east and north, particularly against towns and castles. On June 25, Louis occupied Winchester. Barons who had been until now faithful to the king began to come in and join the French as their rapid advance threatened their estates; among them was even John's brother, the Earl of Salisbury. Early in July Worcester was captured and Exeter threatened, and John was forced back to the borders of Wales. This marks, however, the limit of Louis's success. Instead of pushing his advance rapidly forward against the one important enemy, the king himself, he turned aside to undertake some difficult sieges, and made the further mistake of angering the English barons by showing too great favour to his French companions. Dover castle seemed to the military judgment of the French particularly important as "key of England," and for more than three months Louis gave himself up to the effort to take it.