What was the historical reason for the separation of powers?

The separation of powers is a political principle in some forms of government, in which the legislative, executive and judicial powers of the State are exercised by different government bodies, independent of each other. This is the fundamental quality that characterizes democracy.

Montesquieu argued that "every man who has power is inclined to abuse it; He goes until he finds limits. In order that power can not be abused, it is necessary for the disposition of things to detain power". In this way, the vigilance of the three powers among themselves is entrusted, since each one of them supervises, controls and stops the excesses of the others to prevent, by own ambition, that one of them predominates over the others. It can be contrasted with the merger of powers and separation of functions in parliamentary systems, where the executive and the legislature are unified, because the legislature appoints the executive.  

This doctrine does not refer only to the separation and balance of the three classic powers of the Government, but to the need to divide the political power where it is located, be it in the municipal, regional or national sphere, in order to guarantee political freedom and avoid abuses of power, through the surveillance and reciprocal control of separate powers.

i really really really really really really dont know

c. the dred scott v. sandford case,