Montesquieu, a French philosopher and political thinker, developed the influential theory of separation of powers. He argued that government should be divided into legislative, executive, and judicial branches to prevent tyranny and protect individual liberty. Montesquieu's ideas had a profound impact on modern democratic systems, particularly the U.S. Constitution. His concept of checks and balances ensures that no single branch becomes too powerful, fostering accountability and safeguarding citizens' rights.