July and August were bad months on the frontiers. Within three weeks Mainz, the symbol of previous successes, capitulated to the Prussians, and the Austrians captured the fortresses of Condé and Valenciennes and invaded northern France. Spanish troops crossed the Pyrenees and began advancing on Perpignan. The Piedmontese took advantage of the diversion of republican forces at Lyons in order to invade France from the East. In Corsica, Paoli's revolt expelled the French from the island with British support. British troops opened the siege of Dunkirk in August and in October the Allies invaded Alsace. The military situation had become desperate.