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How did Enzio’s claim to Sardinia worsen Frederick II’s conflict with the pope?

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The conflict between Frederick II and the papacy reached a definitive breaking point in 1238 due to Frederick’s actions regarding Sardinia. The papacy maintained a long-standing claim of suzerainty over the island, viewing it as a papal fief.12

When Frederick’s natural son, Enzio, married the Sardinian heiress Adelasia and was subsequently designated as the king of Sardinia, the Pope perceived this as a direct encroachment on ecclesiastical authority and a strategic threat. This move exacerbated existing tensions, as Pope Gregory IX already deeply distrusted the emperor’s political and religious intentions, fearing that the Papal States were being isolated and encircled by imperial influence.12

Consequently, this act served as the catalyst for the final rupture between the two powers, leading Gregory IX to excommunicate Frederick II for the second time on March 20, 1239, marking the beginning of the last, intense phase of their struggle.12