US States are sometimes described as independent nations inside a federation. Presumably there is trade between these states and there must also be trade deficits and surpluses in these trades. The current Trump administration seems to (incorrectly) relate deficits (but not surpluses) to tariffs, so the logical extension of this would be for US States to impose draconian tariff systems on each other.
Is there any prohibition on US States imposing tariffs on each other?
A state may not impose tariffs on goods from another state.
The allocation of legislative authority over interstate commerce to the federal government, and the associated understanding of a "Dormant Commerce Clause", prevents states from enacting such tariffs. See Tennessee Wine and Spirits Retailers Association v. Thomas, 588 U.S. ___ (2019):
This ‘negative’ aspect of the Commerce Clause” prevents the States from adopting protectionist measures and thus preserves a national market for goods and services.
Congress is also limited in this regard. Article 1, Section 9 of the Constitution reads in part ("export and tax clause" and the "ports clause"):
No Tax or Duty shall be laid on Articles exported from any State.
No Preference shall be given by any Regulation of Commerce or Revenue to the Ports of one State over those of another: nor shall Vessels bound to, or from, one State, be obliged to enter, clear, or pay Duties in another.
