I once read in this book about William Marshal (knight who served Henry the Young King, Henry II, and Richard I). When it got to the part about Phillipe Augustus tearing apart the "Angevin Empire," it stated that contemporaries concluded that he had no chance had the Plantagenets stood together instead of bickering. For example, the Norman levies actually beat back the Capetian army, relieving three sieges of castles, before some of the lords deserted to Richard.
Later on, when the book covered King John's time (aka, epic fail), it mentioned that contemporary chroniclers in the Holy Roman Empire, Paris, and Normandy also thought that had John not caused so much trouble during Richard's reign, the Plantagenets would have easily parrayed the Capetians. They even mentioned that had John stayed faithful to his brother, the situation was strong enough that even he couldn't mess it up.
Is this true? I'm not entirely convinced.
Arguing in favor of contemporary chroniclers, the Plantagenets were on the defensive, castles were a great force multiplier. Henry II had a huge warchest saved up over the years and Richard's crusading didn't eliminate all of it (it might have if Richard stayed longer which is why he tired to raise more money when he left because he might have needed it). The Brittany, Aquitaine, and Anjou had been exempt from tax from the crown... for no good reason. Apparently some pre-1000 king gave them exemption indefinitely, meaning these lands paid nothing to the crown and didn't owe anything financially (heck Aquatine was practically independent) until they were pried from the Plantagenet hands. Thanks to incompetent tax collection, this meant not only Henry II of England was one of the wealthiest men in Europe, his French holdings gave him more income than the Capetians had income.
There was also this place called Chateau Gaillard. Phillipe had pretty much no way of incurring into Normandy with a large footbound army if he didn't go through it. In OTL, the defenders prepared for a siege to last as long as three years, having stored enough grain, but they got beaten anyways. Part of it was because John was wasting time trying to raise funds for mercenaries. part of the reason is that they didn't hold out for three years since John's addition of a poop chute allowed a Captian soldier to get in and open the gates. My point being, it was a really formidable castle and if Richard wasn't on the defensive for so many years, it Phillipe might be too busy with the boarder areas he took early in OTL instead of getting right up to Chateau Gaillard immediately.
Against the Plantagenet cause, even if they united once Henry the Young King died, they were still vassals of the French king. This allowed Phillipe to come up with all shorts of shenanigans and justifications to escheat Normandy from them on the flimsiest of reasons. Once the Capetians won, Normandy would be lost and the cultural heart of England's (Anglo-Norman) nobility would be gone. If the Plantagenets win a war, or two,, or three, they can't eliminate the threat to Normandy. They only got their French lands due to marriage. And until Edward III (after John already lost Normandy), they have no royal French blood.
So what do you guys think? Can the Plantagenets keep their French lands in their family if they stop infighting once Henry the Young King dies? I don't mean all together indefinitely. Henry II planned to divide up his holdings (he didn't think of them as one continuous block) leaving only England and Normandy with the senior line, but Henry the young King's death screwed that up.