The single term "sovereignty" is used to denote two distinct (although related) concepts of constitutional significance.
It refers both to the autonomy of a state with respect to its legislative JURISDICTION and to the supreme authority within the state. There are historical reasons why the same term is used for both, but to confound them is a serious and all-too-common error. The term itself comes from the Latin superans (meaning "rising above" or "overcoming") through the French souverain.
Sovereignty, in the first sense, is a concept derived from international law. A state is sovereign if it is independent of other states and possesses the authority to determine its relationship to other states and to regulate its own internal affairs. Sovereignty, in this sense, is the essential condition required for membership in the family of nations. Sovereign states do not ordinarily make treaties or wage formal war except with other states recognized as sovereign. International law also recognizes some communities as semisovereign, that is, as possessing certain, but not all, of the attributes of sovereignty. The member states of a federal union are in this category.
Internally the several states of the United States are legally sovereign in this sense insofar as they possess jurisdiction, the legitimate authority to declare the law within their territory. But this sovereignty is not unlimited. As the Supreme Court said in PARKER V. BROWN (1943), "The governments of the states are sovereign within their territory save only as they are subject to the prohibitions of the Constitution or as their action in some measure conflicts with powers delegated to the National Government, or with Congressional legislation enacted in the exercise of those powers." The jurisdiction of the states is constitutionally limited by subject as well as by territory, but within their sphere the state governments are as supreme as the national government is within its sphere. This is the meaning of what JAMES MADISON in THE FEDERALIST #39 called the "compound republic."
The states enjoy some other attributes of sovereignty: they may not without their consent be sued in their own courts or in the courts of the United States (see SOVEREIGN IMMUNITY; ELEVENTH AMENDMENT) and they possess independent and plenary authority to lay and collect taxes on persons, things, or transactions within their jurisdiction. Among themselves, also, the...