When the framers drafted our national Constitution in 1787, their use of federalism made republican government possible on a scale large enough to include each of the states. Until then, there were two main ways of governing territories. A unitary system consolidates all sovereign powers into one central government. In such systems, some powers can be devolved to lower regional layers of administration, but each is responsible to the layer above it, and all are ultimately responsible to the one central government. Great Britain is an example of a unitary system.[i] The other form of government is a confederation where all power originates in the individual provinces or states and where a weak central government has only those powers the states explicitly delegate to it. The thirteen colonies established such a confederation after the War of Independence and fairly quickly replaced it with the federal system we have today. In contrast to the unitary and confederation models, U.S. federalism divides and shares powers between the federal government and the states. Each state has all powers not explicitly delegated to the federal government or reserved to the people.
[i] See https://en.wikipedia.org/wiki/Federalism_in_the_United_Kingdom
Image: McConnell’s Historical Maps, US Library of Congress