Question

From whom did modern democracies learn how to safeguard against tyranny by dividing the government's power among different parts?

Answer 1

Well, first of all, I question the assumption in this question -- that modern democracies HAVE learned that lesson. Almost all modern democracies are parlaimentary systems which do NOT divide the government's power. That's why, among other things, you can have European democracies in which monetary union with the eurozone occured in the face of majority opposition from the voters. A great deal of fairly tyrannical from the top governance takes place in a number of modern democracies.

You should not overlook how exceedingly unusual the American system actually is. I am not sure it is duplicated in all of its republican caution -- Federal system, with the central government restricted by a written Constitution, divided and co-equal branches, Bill of Rights, common law tradition, et cetera -- anywhere in the world. Indeed, that's very likely why the United States remains in the historically unique position of being a very powerful country but also one in which individual liberty is extremely high. Usually those two things are in opposition, since the large amounts of central power required to be a big actor on the world stage are usually inevitably applied also domestically, to "fix" things and "make the trains run on time." The 20th century is full of examples.

Certainly the most famous Founder associated with the idea of separation of powers was James Madison, who wrote much of the Constitution, and who explored the importance of dividing power in Federalist No. 51, one of the essays he wrote arguing for adoption of the Constitution.