Peace of Augsburg

The Peace of Augsburg, signed on 25 September 1555 at the Imperial Diet of Augsburg, was a settlement between Emperor Ferdinand I (acting for his brother Charles V) and the Imperial Estates of the Holy Roman Empire. It formally recognized Lutheranism alongside Roman Catholicism within the Empire, ending nearly three decades of intermittent religious warfare that followed Martin Luther's 1517 break with Rome and the 1530 Augsburg Confession.

Its central principle is captured by the later Latin phrase cuius regio, eius religio ("whose realm, his religion"): each secular prince could determine whether his territory would be Catholic or Lutheran, and subjects who disagreed were granted a limited right of emigration (ius emigrandi). Free Imperial Cities with mixed populations were generally required to tolerate both confessions.

The Peace contained important limits that later proved unstable:

• Only Lutheranism and Catholicism were recognized. Calvinists, Anabaptists, and other Reformed groups were excluded, a gap that contributed to later conflict.
• The Ecclesiastical Reservation (Reservatum Ecclesiasticum) required Catholic prince-bishops who converted to Lutheranism to forfeit their offices and lands, preserving the Catholic character of ecclesiastical territories.
• A separate Declaratio Ferdinandea gave some protections to Lutheran knights and towns in ecclesiastical principalities.

The settlement held for over sixty years but its omissions, disputes over secularized church lands, and the rise of Calvinism in territories like the Palatinate eroded it. Tensions culminated in the Thirty Years' War (1618–1648), which the Peace of Westphalia ended by extending recognition to Calvinism and reaffirming territorial sovereignty in religious matters.

For IR scholarship, Augsburg is often cited as an early step toward the Westphalian model of sovereign, territorially defined states, though it operated strictly within the constitutional framework of the Holy Roman Empire rather than between fully sovereign powers.