Federalism
American federalism resulted from circumstances in the United States in the year 1787 rather than an abstract theory. Federalism as a theory of government emerged after the Framers wrote the Constitution. 

▪ Many realized that the weak confederation, under the Articles, was an insufficient system of government.

▪ They perceived that centralized or unitary government (then the pattern in nearly all European states) had its grave faults.

▪ The primary reason why the Framers chose a federal system was that the federal arrangement was just what the American people wanted, and needed, in a very practical sense, in 1787.

Dual Sovereignty
The Constitution allocated some powers to the Federal government, and guaranteed that all other political powers would be reserved to the States or to the people in those States. This division of powers, or “dual sovereignty,” though hotly debated during 1787–1788, was accepted by the States when they ratified the Constitution.

A democratic republic is based on government by the people. But the people govern through their States, not en masse. In this sense they share sovereignty with the States. The American republic is therefore both a democratic republic and a federal republic.