Well, a century before the Hundred Years' War, France did actually invade England. It was 1216, and less than a year earlier King John had rescinded the Magna Carta, and began ravaging the lands and besieging the castles of the barons responsible for it with his large army of Poitevin, Flemish and Danish mercenaries. The barons decided to overthrow him. However, they had no obvious pretender to rally around - John's eldest son, the future king Henry III, was barely 9 years old at the time, and John's nephew Arthur had disappeared in 1202 in mysterious circumstances (presumably murdered by his uncle). They needed a competent adult, preferably someone who knew a thing or two about military leadership and had some connection to the Plantagenet dynasty. So they looked across the English Channel and found their man in Prince Louis of France, the son of John's arch-nemesis Philip Augustus. Louis was exactly what they wanted - he was married to Blanche of Castile, a granddaughter of Henry II and Eleanor of Aquitaine; he was an energetic 28 year old warrior who had gained plenty of military experience both from his father's conquest of Normandy in 1202 - 1204 and the Albigensian crusade; and he had a strong reputation for being chivalrous, morally upright, just and a man of his word, qualities which would undoubtedly endear him to the political community in England. It must be remembered that at this time, England was still a quasi-elective monarchy - while the king had to be related by blood or marriage to the previous king, he didn't have to be the eldest son and the political history of England from 1066 to 1216 essentially reads like one succession dispute after another; the most famous was of course "The Anarchy" of 1135 to 1152, in which daughter and nephew of the late king Henry I battled for the throne.

The invasion was, to begin with, a great success. Louis successfully assembled an army, built a fleet and sailed over to England, arriving there on 21 May 1216, and was immediately joined by half the barons in the kingdom. The citizens of London welcomed him in as well, allowing him to take control of the machinery of government and issue letters telling all the remaining lay and ecclesiastical magnates in the kingdom to give him homage and many did, including King Alexander II of Scotland himself. King John fled west rather than fight him, and so half the kingdom simply fell into Louis' hands. What ultimately thwarted the invasion was an accident that should have been advantageous to his cause - in October 1216 John died. The few bishops and barons that remained ardent royalists moved hastily from Worcester to Gloucester Abbey, where they had the 9 year old Prince Henry crowned Henry III. Though Louis could have done very little to stop this, it was terrible for his PR - he'd now gone from heroically liberating the country from a tyrant to being a foreign bully attacking a young boy who'd done nothing wrong himself. The barons were probably also coming to realise that Louis was had an independent will of his own, and so wasn't guaranteed to rule in their interests. In May 1217, one of Louis' armies was defeated decisively at the siege of Lincoln, which had successfully held out under the leadership of Nicola de la Haie (a woman in her late 60s, widow of the late constable of Lincoln castle) until it was relieved by the internationally famous knight William Marshal, who was acting as Henry's regent. The other French army was defeated at Dover and when Louis' wife Blanche sent reinforcements, they were defeated by Hubert de Burgh, earl of Kent, and Philip d'Aubigny, commander of the south-eastern coast, at a naval battle at Sandwich. Louis was forced to return home, and so the French failed to take England.

As for why the French didn't try to do this again in the Hundred Years War. Well, then the casus belli and strategic objectives were, quite simply, very different. The French kings justified the war as them disciplining a rebellious subject (the king of England in his capacity as duke of Aquitaine) - the war had after all begun because of the disputes between the kings of England and France over sovereignty over Gascony, as well as English support to the rebels in Flanders. Invading England wasn't that much of priority of French kings at all - much more pressing was reclaiming the territories in France they saw as rightfully theirs from the English.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!