Constitutionally, the law denies the right of any state to conduct international relations outside the purview of federal authority. It also rejects the interference of the state in a number of other legal, economic and fiduciary roles, most commonly ones in which continuum of action is required across state lines.
With regard to international relations, the Constitution specifically denies states the right to compose any treaties, alliances or other arrangements of confederation with any foreign entities of their own agency. The most historically significant threat of this kind arose during the Civil War, when the Confederacy courted foreign aid, particularly from Britain and France, for its war effort. Additionally, the state has no right to turn over a fugitive to a foreign authority without the consent of the State Department.
The Constitution denies states the right to produce their own coinage or currency; all currency and coinage must be produced by and disseminated from federally authorized mints and accompanying institutions. States cannot tax or apply similar duties to imports or exports, as this is the province of federal trade authorities. Similarly, states may not alter or in any way undermine the legitimacy of contracts or obligations derived from their terms. Finally, states do not have the right to deny citizens the benefits of due process or any of the other basic protections furnished through the Constitution and subsequent amendments.