According to the Intergovernmental immunity doctrine both the federal government and the states are treated as independent sovereigns and therefore neither sovereign should intrude on the other in certain political spheres. It prevents federal government and individual state governments from intruding on each others sovereignty. This is a principle established under Constitutional law.
The principle of intergovernmental immunity was established by the U.S. Supreme Court in McCulloch v. Md., 17 U.S. 316 (U.S. 1819) wherein it was held that states may not regulate property or operations of the federal government. This doctrine is frequently applied in taxation cases. For example, in Davis v. Michigan Dep't of Treasury, 489 U.S. 803 (U.S. 1989), the court held that Michigan Income Tax Act violated the principles of intergovernmental tax immunity by favoring retired state employees over retired federal employees. In this case state of Michigan violated law when it exempted state and local government pensions from taxation but levied taxes on federal government pensions.