The siege began on 20 March 1631 and Tilly put his subordinate Gottfried Heinrich Graf zu Pappenheim in command while he campaigned elsewhere. After two months of laying siege, and after the fall of Frankfurt an der Oder to the Swedes, Pappenheim finally convinced Tilly, who had brought reinforcements, to storm the city on 20 May with 40,000 men under the personal command of Pappenheim. The assault was successful and the walls were breached, but the commanders supposedly lost control of their soldiers. A massacre of the populace ensued in which roughly 20,000 of the 25,000 inhabitants of the city perished by sword and the fire which destroyed most of the city, then one of the largest cities in Germany and about the size of Cologne or Hamburg.