The Imperial Troops under Tilly Storm Magdeburg (1631) The city of Magdeburg, which had declared itself Protestant in 1524 and joined the Schmalkaldic League in 1531, became a center of Protestant opposition to the Counterreformation. During the Thirty Years War, Imperial troops stormed the city after a ten-day siege on May 20, 1631. Around 20,000 residents perished and Magdeburg became one of the most poignant examples of the devastation wrought by the Thirty Years War. Colored copperplate engraving, unknown artist, 1631.