In Defensor pacis, Marsilius sought to demonstrate, by arguments from reason (in Dictio I of the text) and by argument from authority (in Dictio II) the independence of the Holy Roman Empire from the Papacy and the emptiness of the prerogatives alleged to have been usurped by the Roman pontiffs.

Relying heavily on Scripture, Marsilius seeks to show that Jesus did not claim to possess any temporal power and that he did not intend his church to exercise any.

[11] Today, Marsilius's Defensor pacis is best remembered not for its theology but for its political philosophy and legal theory.

Marsilius agrees with Aristotle that the purpose of government is the rational fulfillment of humans' natural desire for a "sufficient life".

[12] However, he goes beyond Aristotle in embracing a form of republicanism that views the people as the only legitimate source of political authority.

[13] Marsilius and John of Jandun, who has sometimes been credited as a co-author of Defensor pacis, left France for Louis' court in Bavaria.

Others were also under his protection, including Michael of Cesena and the philosopher William of Ockham, an advocate of an early form of church and state separation.

In January 1328 Louis entered Rome and had himself crowned emperor by the aged senator Sciarra Colonna, called captain of the Roman people.

Three months later, Louis published a decree declaring "Jacque de Cahors"—Pope John XXII—deposed on grounds of heresy.

In recompense for his services, he was appointed archbishop of Milan,[14] and John of Jandun obtained from Louis IV the bishopric of Ferrara.

This work, and Marsilius's variation, sought to justify the exclusive jurisdiction of the emperor in matrimonial affairs: Louis of Bavaria had recently annulled the marriage of the son of the King of Bohemia.