Kings of France, at various times in the middle ages, decidedly lacked the raw power to force unruly subjects to obey. At their low point, ca. 1000, they were barely able to control the region right around Paris, let alone stand up to the likes of the Duke of Normandy, the Count of Anjou, or the Duke of Aquitaine, among others.
But military power was only part of the story. The great nobles, even while flouting the king in practice, continued to assert their loyalty to the crown, and the kings continued to find ways to circumvent the practical limitations of military dependence and to assert their authority in other ways. Whenever the great nobles were in dispute with each other, and whenever one found himself at odds with his own subordinates, the king was always a party to the conflict, ready to mediate as an impartial lord or to apply whatever leverage he could (men, money, or moral authority) to shift the outcome in his favor. And they were generally very successful at it.
And why did the king retain the ability to extract allegiance from nobles who were more powerful than him, and to intervene in disputes where he really had no direct stake or jurisdiction? The one's easy - because the king was the KING. The whole point of a king in the medieval mind was that he was placed by God over the kingdom, sanctified by God through the rite of anointment at his coronation, and therefore, indisputably, he was NOT just another noble. Overt rebellion against the king was the same as rebellion against God. When the king came into the picture and asserted his authority as king in a matter, everybody else had to either put up or shut up.
The brighter kings recognized that they shouldn't push their positional power too much further than they had the actual force to back it up, so in a lot of dealings with their nobles - even the ones that involved armies and battles - they didn't necessarily invoke the "I'm the King and God says I'm better and badder than you" argument till they were already winning.
Because winning was in itself a proof that God favored the King. Once they had the momentum, no one could stop it.
After 1066 the kings of England borrowed many elements of royal ritual modeled in France to enhance their own position. "Anointed by God" gave a king a demonstrable advantage over un-anointed nobles, even though the English crown was never quite a secure as the French. Because the French also benefited from a dynastic miracle - 300 years of unbroken succession from father to eldest son - while the English crown passed by right of conquest more often than not.