Britannica Sources used in this answer

Why did Frederick I Barbarossa attack Milan?

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People walk and shop inside the ornate Galleria Vittorio Emanuele II in Milan, Italy, with its glass-domed roof, decorative mosaics, and luxury store entrances. — Milan Shoppers in the Galleria Vittorio Emanuele II, Milan.

Frederick I Barbarossa attacked Milan primarily to restore and enforce his imperial rights and to bring the city under the direct authority of the Holy Roman Empire.12

During the 12th century, Milan had established itself as a powerful, autonomous commune and engaged in aggressive conflicts with neighboring cities, such as the destruction of Como and Lodi. Frederick viewed these actions and the city's growing independence as a challenge to his imperial jurisdiction. His broader policy aimed to create a well-controlled system of cities and castles governed by imperial officials, which would also provide the empire with significant tax revenue, such as the fodrum levy.21

Milan’s resistance to these imperial demands led to a nine-month siege, resulting in the city's surrender and the destruction of its fortifications in 1162. This conflict was part of a larger struggle between the Holy Roman emperors and the northern Italian communes, which eventually led to the formation of the Lombard League to defend communal liberties.23