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Why did William I give qualified support to the Union of Utrecht in 1579?

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Portrait of William I, Prince of Orange, by Adriaen Thomasz Key. He wears a dark embroidered coat, a white ruff collar, and a black cap, with a trimmed beard and mustache. — William I Portrait of William I, oil on panel by Adriaen Thomaszoon Key, c. 1579; in the Rijksmuseum, Amsterdam.

William I, Prince of Orange, gave only qualified support to the Union of Utrecht because it undermined his long-standing political strategy for the Low Countries.12

William’s primary objective was to maintain a broad, unified resistance against Spanish domination that included both Roman Catholics and Calvinists throughout all the provinces. He believed that the success of the rebellion required mutual toleration between these religious groups.12

In contrast, the Union of Utrecht moved away from the religious settlement established by the Pacification of Ghent and toward a monopoly of public religious practice for Calvinists in key provinces like Holland and Zeeland. By creating a "closer union" that was parallel to and effectively opposite of the pro-Spanish Union of Arras, the agreement threatened to fracture the broader coalition William had struggled to keep intact. Consequently, William continued to work for another half decade to preserve a wider union and secure international support, even as the Union of Utrecht laid the foundation for the separate Dutch Republic.12