I knew that in the centuries following the Norman conquest of England in 1066, English kings spent quite of bit of time on the continent, running, fighting for, or trying to reconquer their lands in Normandy, Aquitaine, Maine, and Anjou. But I didn't realize that the French had ever landed on English soil with conquest in mind.
According to a biograph of William Marshall that I am reading, when King John died in 1216 and left a 9-year-old son (Henry III) as his heir, the English nobles did not automatically transfer their allegeince to the boy. In fact, many of them, already in rebellion against John, had chosen the French prince, Louis, to be their leader and had invited him to be the next king of England. So in 1217, Louis and his knights landed in England and soon controlled southeastern England, with the exception of Dover and Lincoln castles; it took William most of the summer and fall to run the French out and regain the loyalty of the rebel nobles. Since the French king, Phillip Augustus, had already run John out of Normandy and his other possessions, this was the beginning of the true separation between France and England, although the English kings tried unsuccessfully to reverse it for another couple centuries.