Who was the major proponent of the theory of the separation of powers?

The major proponent of the theory of the separation of powers was Montesquieu. In his influential work "The Spirit of the Laws," Montesquieu articulated the idea that government should be divided into distinct branches—legislative, executive, and judicial—to prevent any one branch from wielding too much power and to promote a system of checks and balances. This framework was foundational in shaping modern democratic governance and was notably incorporated into the structure of the U.S. Constitution. His analysis of the separation of powers emphasizes the importance of this division in safeguarding political liberty and preventing tyranny, making his contributions critical to the development of democratic theory and practice.