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Why did the dispute over Milan worsen relations between Gregory VII and Henry IV?

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Gregory VII excommunicating Henry IV — St. Gregory VII Pope Gregory VII, after his expulsion from Rome, laying a ban of excommunication on the clergy “together with the raging king” (Henry IV of Germany), drawing from the 12th-century chronicle of Otto of Freising; in the library of the University of Jena, Germany.

The dispute over Milan became a central point of contention between Pope Gregory VII and Henry IV because it directly challenged the authority of the church versus the monarchy regarding the appointment of clergy, known as lay investiture.12

Initially, Gregory and Henry reached an agreement to settle the Milanese problem, with Henry renouncing his right of investiture. However, after defeating the Saxon rebellion, Henry felt empowered to break this agreement by nominating his own court chaplain as the archbishop of Milan. This violation of their pact caused Gregory to question the king's trustworthiness and led to a series of escalating confrontations.12

When Gregory warned Henry against this behavior, the king responded by attempting to depose the pope at the synod of Worms in 1076. This impulsive reaction transformed a specific disagreement over Milan into a fundamental conflict over the relationship between church and state, ultimately leading to Henry's excommunication and the broader struggle for dominance between priestly and royal powers.12