Relations between the French revolutionaries with neighbouring monarchies had deteriorated following the Declaration of Pillnitz in August 1791. Eight months later, following a vote of the revolutionary-led Legislative Assembly, France declared war on Austria on 20 April 1792; Prussia, having allied with Austria in February, declared war on France in June 1792. In July 1792, an army under the Duke of Brunswick and composed mostly of Prussians joined the Austrian side and invaded France. The capture of Verdun (2 September 1792) triggered the September massacres in Paris. France counterattacked with victory at Valmy (20 September) and two days later the Legislative Assembly, flushed with victory, proclaimed the French Republic.