In 1785, stadtholder William V fled his palace in the west of the country for Nijmegen in the east, as the States of Holland were not willing to send their troops to fight the Patriots. In May 1787, the stadholder's troops were defeated by the militia of Utrecht near Vreeswijk. When Princess Wilhelmina was stopped by patriot militia near Goejanverwellesluis on June 28, 1787, she applied to her brother Frederick William II of Prussia for help. On September 13 a Prussian army of 20,000 men under the command of Duke of Brunswick crossed the border. The fortress of Vianen was deserted, and the city of Utrecht opened its gates. At the fortress of Woerden preparations for defense were made, but there was no actual resistance when the Prussians arrived. In Amsterdam several houses of patriot regents were plundered by mobs. The stadholder returned to The Hague, and Amsterdam, the last city to hold out, surrendered on October 10.