Overtime, the power of states has been eroded, since the economic crash of 1929 and Roosevelt's New Deal policies meant huge growth and role of the federal government
Some individual policy areas are controlled and carried out by both the federal and regional government, creating an interdependence where they have to work together
Federal government usually provides resources for policies they would like to see, though these grants come with a criteria, allowing the federal government to restrict states
Otherwise, federal mandates are passed, forcing states to comply
Sometimes, states independently create laws, making conflict with the federal government challenging these laws