The Battle of Saint-Denis was the last major action of the Franco-Dutch War, which took place on 14 August 1678, four days after France and the Dutch Republic signed the Treaty of Nijmegen. The battle was fought to prevent the French capturing the town of Mons, then on the border between France and the Spanish Netherlands, now modern Belgium. Its result is disputed.
Fighting was concentrated around the nearby villages of Saint-Denis and Casteau. A French army of 40,000 under Marshal Luxembourg was attacked by an Allied Dutch-Spanish army of 45,000 led by William of Orange. A series of Allied assaults were successfully repulsed by the French and William ordered a retreat after six hours of fighting. However, Luxembourg abandoned the siege the next day and Spain retained Mons in the treaty agreed with France on 17 September.