Federalism is a system of government in which power is divided between a central authority and constituent political units, in the case of the United States, between the national (federal) government and the state governments. This system of government was established by the Founding Fathers in the U.S. Constitution, and has been a key component of American governance ever since.

The division of power between the national and state governments is defined by the Tenth Amendment of the U.S. Constitution, which states that any powers not specifically delegated to the federal government by the Constitution, nor prohibited by it to the states, are reserved to the states or to the people. This principle of dual sovereignty ensures that each level of government has its own spheres of authority and responsibility, and serves as a check on the concentration of power in any one entity.

The division of power between the national and state governments is achieved through a system of shared and exclusive powers. Shared powers, also known as concurrent powers, are those that are exercised by both the federal and state governments. These powers include the ability to levy taxes, regulate commerce, and establish courts. Exclusive powers, on the other hand, are those that are reserved either for the federal government or for the state governments. For example, only the federal government has the power to declare war and regulate interstate and foreign commerce, while only the states have the power to regulate intrastate commerce and conduct elections.

In addition to shared and exclusive powers, the U.S. Constitution also establishes a system of checks and balances between the national and state governments. This ensures that no one level of government becomes too powerful, and that each level of government is able to hold the other accountable for its actions. For example, the Supremacy Clause of the Constitution declares that the Constitution, federal laws, and treaties are the supreme law of the land, thereby giving the federal government the authority to preempt state laws that are inconsistent with federal law. Likewise, the federal government is prohibited from interfering in certain areas of state authority, such as the regulation of intrastate commerce.

Overall, federalism is a critical component of American governance and has played a key role in shaping the relationship between the national and state governments. By dividing power between these two entities and establishing a system of shared and exclusive powers, the U.S. Constitution ensures that each level of government has its own spheres of authority and responsibility, while also limiting the concentration of power in any one entity. This system of dual sovereignty has helped to preserve the balance of power in American government and ensures that the rights and interests of the people are protected.