Why was the Union of Utrecht important to the Dutch revolt against Spain?

The Union of Utrecht, concluded on January 23, 1579, was a pivotal agreement that served as the foundation for the Dutch Republic and a "makeshift constitution" for the United Provinces until 1795.12

Its importance to the revolt against Spain included:

• Military and Political Consolidation: It established a "perpetual" military league among northern provinces to resist Spanish rule. While provinces remained sovereign in internal affairs, they acted as a unified body regarding foreign policy, war, and federal taxation.1
• Separation from the South: By complementing the Union of Arras—which saw southern provinces reconcile with King Philip II—the Union of Utrecht effectively split the Low Countries, solidifying the northern resistance.1
• Shift in Religious Policy: It moved away from the broader religious toleration sought by William I, favoring the dominance of Calvinism in key provinces like Holland and Zeeland.1

Johan van Oldenbarnevelt was a key negotiator of this treaty, working to ensure that Holland secured a dominant political position within the new union.2