In the time of Charlemagne one was obliged under great penalties to show up when called to any war whatever ; no excuses were accepted, and the count who exempted anyone would himself have been punished.  But the treaty of the three brothers  placed a restriction on this  which, so to speak, took the nobility out of the king’s hand : they were no longer obliged to follow the king to war except when that war was defensive. They were free, in others, to follow their lord, or to attend to their business.
The death of a hundred thousand Frenchmen in the battle of Fontenay  made what was still remaining of the nobility think that they would ultimately be exterminated by their kings’ private wars over their divisions, and that their ambition and their jealousy would cause the spilling of all the blood still left to shed. They made the law that the nobility would be required to follow princes to war only when it was a matter of defending the state against a foreign invasion. It was in effect for several centuries.