During the Hundred Years’ War, God told Joan of Arc to lead the French people to victory against the English Invaders.
At this point, England and France were both Catholic Countries. It is not like the French were fighting off non-Christians. Both the French and the English believed in Jesus Christ.
The only explanation I can come up with is that if France was conquered, than the French Catholics would have eventually been persecuted (like the English Catholics were) when the Protestant Reformation comes to England later down the road.
Is there something about the Hundred Years’ War I am missing?