How did France became a constitutional monarchy?

The National Assembly, under the leadership of Abbe Sieyes and Mirabeau, had resolved to draft a Constitution for France which would effectively limit the powers of the monarch. These powers, instead of being concentrated in the hands of one person, were now separated and assigned to different political institutions - the legislature, executive and judiciary. The monarchy was retained, but the king did not enjoy absolute power and authority, as he did in the days of pre-revolution France. Therefore, through the new Constitution, the leadership of the Third Estate transformed France into a constitutional monarchy.