Article III, Section 2, Clause 2:
In all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be a Party, the supreme Court shall have original Jurisdiction. In all the other Cases before mentioned, the Supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make.
It seems settled, though not without dissent, that state courts have no power to enjoin proceedings 1 or effectuation of judgments 2 of the federal courts, with the exception of cases in which a state court has custody of property in proceedings in rem or quasi in rem, where the state court has exclusive jurisdiction to proceed and may enjoin parties from further action in federal court. 3