When war broke out in 1756, Frederick overran Saxony and campaigned in Bohemia, where he defeated the Austrians on 6 May 1757 at the Battle of Prague. Learning that the French forces had invaded his ally's territory of Hanover, Frederick moved west. On 5 November 1757, an infantry regiment of about 1,000 men and 1,500 of his cavalry defeated the combined French and Austrian force of 30,000 at the Battle of Rossbach in a 90-minute battle. In his absence, however, the Austrians had managed to retake Silesia: the Empress's brother-in-law, Prince Charles, took the city of Schweidnitz and moved on Breslau in lower Silesia.