Hurrah! A question I can help with.

Unfortunately your textbook is summarizing a conflict that is pretty complex and assigning Henry far more agency than he probably had. Let me explain.

Henry III was the son of King John of Magna Carta fame - the same John who was so reviled by his barons that they rose in revolt and caused him to sign the Great Charter, thereby limiting the power of the English Monarchy. There was a LONG way to go before we arrived at anything recognizable to the Parliamentary state we know today, but it was no doubt a crucial step.

But why did they hate him so much? Well John, the younger brother of Richard the Lionheart, had come up against the brilliant Phillipe Augustus - one of the most powerful and frankly intimidating French Kings, who set about dismantling the territories controlled by the English King in France - nameless the rich duchy of Normandy (from whence had come William the Conqueror), Anjou (seat of John’s father’s family, the Plantagenet dynasty) Touraine (the area around the key town and fortress of Tours) Poitou, Brittany and Gascony (the area around the towns of Bordeaux and Bayonne, but much expanded in King Henry II’s time). John had lost the respect of other feudal lords (see Arthur of Brittany) and many of his own, and was frankly outwitted and outmaneuvered by the French King. Normandy, Anjou, Touraine and Brittany were wrested from Johns hands, with his control limited to Gascony and parts of Poitou by the end of his reign. His enormous demands for funding a war to reconquer the lost regions was a key element in bringing about the rebellion of his barons and the ensuing Barons War that eventually led to John’s death on the campaign trail.

So we arrive at Henry III, the infant son who was proclaimed King by John’s followers (among whom remained the formidable William Marshal) and in whose name the royalist eventually wrested control of England. But you can see the issue - England May have been retained, but a majority of Henry’s lands on the continent had now been in French hands for a good while. By the time Henry was old enough to begin campaigning on his own, Normandy, Anjou, Touraine and Brittany were far from his grasp. To his credit he campaigned in Poitou to regain elements of that region but his army was too small, too ill disciplined, his coffers were soon empty and the only good to come of it was English soldiers teaching some allied French troops how to chug (seriously, you can check that out).

Henry continued to claim the title of Duke of Normandy, Count of Anjou etc. But eventually was content to settle for Gascony, which had by this time become the only legitimate English possession in France. When France continued to encroach on this last bastion...well then you have the lead up of the Hundred Years War.

In short, by the time Henry III became King his father had already bumbled away most of the continental possessions through failed war against France and the rebellion of his own barons. Henry’s reduced Kingdom was too small, too poor and too isolated to put up much of a fight until a century later when military technology shifted to give the English a more level playing field against French might.

Let me know if you have any questions!

Some helpful sources for the time period:

Tout, The Political History of England, 1216–1377 (1905)

Barlow, the Feudal Kingdom of England, 1042-1216

Prestwich, Plantagenet England, 1225-1360 (2005)