Federalism is one of the central principles of American Constitutionalism because helps in the dividing and sharing of power between state and national governments, thus, making organization of the government manageable and standardized.
The other major principles of American Constitutionalism are:
Popular Sovereignty; Checks and Balances; Limited Power of the Government; Separation of Branches.
They all had essentially the same idea about self-government and people's rights
