Factual error: At one point, Quasimodo says, "At sunset, I ring the evening mass, and after that, I clean the cloisters, and then I ring the vespers, and..." In fact, evening masses were not permitted by the church until the twentieth century.
Revealing mistake: At the start of the montage in which Frollo ransacks Paris for the gypsy girl, the shot of the three soldiers kicking down the baker's door very briefly places the more distant soldier off to the right "in front" of the closer central soldier; this is a case of incorrect animation cel layering.
Other mistake: During the "Hellfire" sequence, when Frollo is singing "God have mercy on her, God have mercy on me...", his shadow against the wall behind him rises as he backs up closer to the wall. However, when he turns around to press against the wall, his shadow ends up far above his body when it should have become level with his body by that point - like the very next close-up shot of Frollo shows, which is itself a continuity error.
Answer: That's a good question. France actually fought no wars between the Battle of Castillon in 1451, and the Italian Wars which began in 1494. Seeing as the movie takes place in 1482 it seems strange that Phoebus would have fought in any war, unless he is at least 45 years old.