In the E.U., the principle of subsidiarity functions in theory like the Tenth Amendment does in the U.S.—again in theory. In both cases, public authority on a given domain or policy-area is preferentially to be exercised at the state rather than federal level. The principle, while not federalism per se, can be an element of it. Taking subsidiarity to be “really federalism” turns the latter into an alliance—giving the states potentially so much power that the government of the federation or union itself cannot act as a check on the state governments.
