what was the event that started the first war mentioned above?
Relations between the French revolutionaries with neighbouring monarchies had deteriorated following the Declaration of Pillnitz
War of the First Coalition
Relations between the French revolutionaries with neighbouring monarchies had deteriorated following the Declaration of Pillnitz in August 1791. 
what was the rhine campaign of 1796 about? This was the last campaign of the War of the First Coalition, part of the French Revolutionary Wars. which armies were involved in this? First Coalition armies and French Republican armies.
wiki:13516991
War of the First Coalition [SEP] Introduction
The War of the First Coalition () is a set of wars that several European powers fought between 1792 and 1797 against initially the constitutional Kingdom of France and then the French Republic that succeeded it. They were only loosely allied and fought without much apparent coordination or agreement; each power had its eye on a different part of France it wanted to appropriate after a French defeat, which never occurred. 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 Prussia and Austria on 20 April 1792.