- Separation of powers
One of the most important of the basic principles that guided the framers of the US Constitution in their design for America's future governance was the idea that the root cause and essence of tyrranical government is the concentration of control over all the powers and functions of government in the hands of the same individual or narrow political faction. The corollary the Framers drew from this was the separation of powers principle: that free popular government can best be sustained by dividing the various powers and functions of government among separate and relatively independent governmental institutions whose officials would be selected at different intervals and through different procedures by somewhat different constituencies so as to make it unlikely that the same small faction could gain control of them all at the same time. Thus, in the American federal republic the Framers designed, "the power surrendered by the people is first divided between two distinct governments [the Federal government and the governments of the several states], and then the portion allotted to each subdivided among distinct and separate departments [the executive, the legislative, and the judicial]." [Madison, The Federalist #51]
The idea that concentrated political power is a mortal danger to civil liberties and popular rights remains to this day one of the most persistent and characteristic features of American ideologies and popular thinking about politics. In comparison with other advanced industrial countries, the United States possesses one of the most complex governmental structures and perhaps the most broadly diffused distribution of governmental authority among independent agencies. Not only do American governmental arrangements still allocate power to separate executive, legislative and judicial branches at both the state and federal levels, but they also feature a great variety of forms of relatively autonomous and geographically overlapping governmental bodies at the local level -- including not only general purpose county and municipal governments but also a wide variety of functionally specialized mini-governments such as elected district school boards, flood control district boards, water resource planning boards, transit authority boards and the like.