I think commentators here are over-estimating the ability of England, or someone in control of English resources, to stand up to an attack from one of the continental nobles. The continental nobles in what is now France and Germany seem to have been much more advanced militarily. There is a reason why, outside of Scandinavia and Scotland, pretty much all the European ruling dynasties wound up either of French (I'm including Norman) or German descent.
So yes, a Count of Flanders definitely had the resources to conquer England, as much if not more than a Duke of Normandy, a Count of Anjou, or a Count of Blois. The claim to the throne just provides a motivation.
William either would have become King of England or have been assassinated before he could make the attempt.
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Explain then how difficult and grueling a process it was for Matilda to assert her claim, even with many nobles on her side? She had the resources of Anjou, and soon afterward Normandy behind her, so what gives?
William the Conqueror's success in 1066 was a freak accident that almost didn't happen. Solidification of his rule took at least a decade, but by late 1066 he at least had dozens of castles in the south for control.
By 1120, however, there are hundreds of Norman castles all over England. Without some combination of popular, noble, and church support, Clito is screwed. Bishop Henry of Winchester is King Stephen's brother, and Stephen was very generous to the church. I don't see them going Clito's way unless Matilda defeats Stephen clearly first. As mentioned before, the nobility of England owes almost all of its power to the graces of Henry. The powerful marcher lords, in particular, enjoyed his patronage.
As for ruling families, that's silly. France and Germany are much more centrally located than England, and marrying out to the rest of Europe gives an advantage. To Anglo-Norman nobility there was little advantage in marrying outside northern France, Flanders, and Scotland.
England at this time was one of the wealthiest areas of Europe, with a highly developed ruling structure and legal system. The king controlled all the mint and theoretically the nobles were at his beck and call...practical as well as theoretical most of the time after 1070. If such a wealthy nation was truly so easy to conquer, then surely it would have fallen more than...what, thrice? to foreign invaders. It's actually more telling that the English kings were able to keep a presence on the French mainland for 500 years, despite the meteoric rise of French royal power in the late 12th and early 15th centuries.