Question

how was France declared as a constitutional monarchy​

Answer 1

As such, France declared war on Austria in 1792. ... Thus, the Constitutional Monarchy came to an end, and the First French Republic was declared. The French Constitutional Monarchy was something of a failure.

Answer 2

French Constitution of 1791. The short-lived French Constitution of 1791 was the first written constitution in France, created after the collapse of the absolute monarchy of the Ancien Régime. One of the basic precepts of the revolution was adopting constitutionality and establishing popular sovereignty.