Why did Milan pass from Spanish rule to Austrian rule in 1706?

The transition of Milan from Spanish to Austrian rule in 1706 was a direct consequence of the War of the Spanish Succession.1

Following the death of Charles II of Spain in 1700, a major conflict erupted over the succession of the Spanish throne and the distribution of its vast European territories. As the war progressed, military forces moved to secure control over strategic regions. In September 1706, Prince Eugene of Savoy entered Milan and took control as its first Austrian governor, effectively ending the period of Spanish domination that had begun in the 16th century. This shift was later formalized through international treaties, including the Treaties of Utrecht and Rastatt, which established a new balance of power in Italy between the Austrian Habsburgs and other European powers.12