As members of the Union, states have no powers of external sovereignty, but, subject to the Federal Constitution, they possess many of the attributes of internal sovereignty.
As members of the Union, states do not possess external sovereignty. Nevertheless, except as limited by the Federal Constitution, a state does possess internal sovereignty or many attributes thereof, and states are often described as sovereign.
The federal system established by our Constitution preserves the sovereign status of the states. The Constitution established a system of dual sovereignty under which states surrendered many of their powers to the federal government but retained residuary and inviolable sovereignty. Although the Constitution grants broad powers to Congress, our federalism requires that Congress treat the states in a manner consistent with their status as residuary sovereigns and joint participants in the governance of the nation. States retain a significant measure of sovereign authority but they do so only to the extent that the Constitution has not divested them of their original powers and transferred those powers to the federal government.
Sovereignty of each state implies a limitation on the sovereignty of all sister states, a limitation express or implicit in both the original scheme of the Constitution and the Fourteenth Amendment.
No principle of general law is more universally acknowledged than the perfect equality of nations. Each nation legislates for itself, but its legislation can operate on itself alone, and except as otherwise provided by statute or agreement, only within its own territory. Every sovereign state is bound to respect the independence of every other sovereign state.
A nation possessing the attributes of sovereignty is vested with all the powers of government necessary to maintain effective control of foreign relations. It may, for example, enter into compacts or treaties, exclude aliens, and enter into diplomatic relations with other nations. The power of our Federal Government to conduct foreign affairs is implicit in its sovereignty, and such power is necessary, complete, and exclusive.