Why did the Union of Utrecht weaken cooperation between Catholics and Calvinists in the Low Countries?

The Union of Utrecht did not inherently aim to weaken cooperation between Catholics and Calvinists; rather, it undermined the broader policy of William I (Prince William of Orange), which sought to foster collaboration between the two groups to resist Spanish domination.1

The Union of Utrecht, signed in 1579, moved away from the religious settlement established by the Pacification of Ghent and toward a clear predominance of Calvinists and their monopoly on public religious practice in key provinces like Holland and Zeeland. Simultaneously, the Union of Arras was concluded, which began the reconciliation of southern provinces with Philip II of Spain.1

These two parallel but opposing unions effectively polarized the Low Countries along religious and political lines, making the mutual toleration required for William’s unified resistance impossible to maintain. Consequently, the northern provinces moved toward a state defined by a Calvinist identity, while the south remained under Spanish and Catholic influence.12