International law has in modern times spoken with no very certain voice as to the permissible treatment of alien enemies found within the territory of a belligerent at the outbreak of war.
There is, however, little doubt that such persons, although now more usually allowed to remain, during good behaviour, may be expelled, and, if necessary, wholesale, as were Germans from France in 1870.

But may such persons be, for good reasons, arrested, or otherwise prevented from leaving the country, as Germans were prevented from leaving France in the earlier days of the Franco-Prussian War?
Grotius speaks with approval of such a step being taken, "ad minuendas hostium vires." Bynkershoek, more than a century later, recognises the right of thus acting, "though it is rarely exercised." So the Supreme Court of the United States in _Brown v.

United States_ (1814).

So Chancellor Kent (1826), and Mr.Manning (1889) is explicit that the arrest in question is lawful, and that "the individuals are prisoners of war." Vattel, is it true (1758), ventures to lay down that-- "Le Souverain qui declare la guerre ne peut retenir les sujets de ennemi qui se trouvent dans ses etats au moment de la declaration ...

en leur permettant d'entrer dans ses terres et d'y sejourner, il leur a promis tacitement toute liberte et toute surete pour le retour." And he has been followed by some recent writers.