There is a saying in emergency management that all disasters are local. While the saying is pithy, it embodies the approach used, by emergency management officials in the United States, to manage emergencies and disasters.
When an incident takes place, individuals, communities, and local governments respond to the event. When these local entities become overwhelmed, they depend on the state for additional assistance. When the state becomes overwhelmed, the governor requests support from the federal government.
The above description is both practical and consistent with the reality of responding to incidents (imagine responding to an event in the reverse order). However, the structure also reflects the governing system of the United States that is known as federalism. It is crucial to have an understanding of federalism in order to grasp the legal framework through which emergency management functions are executed.
Federalism is the system of government set up by the Framers of the Constitution in which state governments have a protected existence and can make independent decisions concerning certain realms of governmental activity. This sovereignty cannot be superseded by the federal government unless the Constitution, law, and the federal courts decide otherwise.