As long as we’re quoting Vattel:
It is an accepted maxim of international law that every sovereign nation has the power, as inherent in sovereignty and essential to self-preservation, to forbid the entrance of foreigners within its dominions or to admit them only in such cases and upon such conditions as it may see fit to prescribe. Vattel, lib. 2, §§ 94, 100; 1 Phillimore (3d ed.) c. 10, § 220.
[Via TM]