Complex sentence: When France declared war against Austria on 20 April 1792, Vidocq had to participate in the battles of the First Coalition, including the Battle of Valmy in September 1792.
Split into simpler sentences: When France declared war against Austria on 20 April 1792, Vidocq had to participate in the battles during the First Coalition. For instance, he took part at the Battle of Valmy in September 1792.