The Siege of Allenstein or the Siege of Olsztyn took place from January 1521 to February 1521, during the Polish–Teutonic War (1519–1521). The town of Allenstein (Olsztyn) was defended by Polish forces against those of the Monastic Order of the Teutonic Knights. The defense of the town was successfully organized and coordinated by astronomer and scholar Nicolaus Copernicus. The Polish forces successfully resisted the Teutonic Knights, who eventually had to lift the siege. By the end of 1521 the Teutonic Knights were forced to ask for an armistice and in 1525 the Grand Master of the Order, Albrecht Hohenzollern, paid homage to the Polish king, Sigismund the Old.