Wellington's own army absorbed repeated French assaults for hours before the Prussians reached the field in strength, and accounts differ on whether the Anglo-allied line was already close to breaking the French attack on its own or whether the outcome was decided by the Prussian flank attack that followed. Both readings are represented in the historical record and neither has settled into a single accepted account.
What would resolve this A reconstruction of French troop cohesion and casualty timing precise enough to show whether the French army was already breaking before the Prussian attack struck its flank.
OpenMilitary historyWikipedia: Battle of Waterloo