Why is the Battle of Heiligerlee considered the beginning of the Eighty Years' War?

The Battle of Heiligerlee, fought on May 23, 1568, is considered the beginning of the Eighty Years' War because it marked the start of an organized military invasion of the northern Netherlands led by Louis of Nassau against Spanish rule.12

Following the arrival of the Duke of Alba in 1567 and his subsequent campaign of repression, resistance forces—including the Geuzen—sought to challenge Spanish authority. Louis of Nassau’s victory at Heiligerlee represented a significant, albeit modest, military engagement that signaled the transition from localized unrest to an armed struggle for independence. While the battle itself was a tactical success for the rebels, it was followed by a broader, long-term conflict that ultimately led to the separation of the northern and southern Netherlands and the formation of the United Provinces of the Netherlands.123