§2. There are two ways in which the lines of separation between States lack perfect definiteness; firstly, there is no clear and universally accepted determination of the criteria of membership of a State; and secondly, each State has aliens residing in its territory and thus partially subject to its laws, and also has some of its own members in a similar relation to other States. As regards the first point, there is of course no doubt that persons born within the territory of a State of parents belonging to it---or unknown parents---themselves belong to that State, unless their connection with it has been severed by some act done by it or them, designed to have this effect. But, owing to the extensive interfusion of modern nations, there is a not inconsiderable margin of doubtful cases, as to which different nations take divergent views, and are liable to come into conflict. For instance, European nations disagree somewhat as to the national character properly belonging to children born out of their parents' country. The older view---to which England still adheres in principle---made the place of birth decisive; the newer rule, which since the Code Napoleon has tended to prevail, makes the child follow the nationality of its parents. It seems desirable that liberty of choice should be generally allowed in such cases; but, it is, I conceive, in accordance with the principle of mutual non-interference that each State should have an unquestioned right of determining the relation to itself of children born within its territory; all that can be demanded is that due warning should be given to other nations of the laws laid down by it on this subject.
A more important difficulty is presented by the question whether the members of a community generally should have unlimited liberty of severing their connection with it by expatriation. In practice, as I have before said, free emigration is now generally conceded by European States; but a universal and unlimited right of expatriation is not yet generally admitted; and it is easy to see that where universal military service is compulsory it may be necessary to prohibit temporary emigration, lest it should take place widely with the view of evading military service. It would probably be inexpedient, even in this case, to take any measures to prevent such emigration, beyond the announcement of a penalty to be inflicted if the evasive emigrant returns; but however this may be, I do not think that any measures taken to prevent expatriation can, on the principle of mutual non-interference, be regarded as an offence by any other State, except so far as the persons whose expatriation is prevented are already claimed as subjects by that other State. On the other hand, it cannot be regarded as an interference that a foreign State should admit such an evasive emigrant to membership; but it seems clear that it cannot, consistently with our principle, claim to protect him, if he returns to his native land, from the penalty incurred by unlawful emigration.
In any case, we have to recognise that ordinary intercourse brings political societies into a condition of partial interfusion, to which the ordinary intercourse of individual human beings offers no parallel; in consequence of which each modern State contains a number of aliens residing temporarily or permanently within it. But on the principle that limits strict duty to non-interference, it must be competent for a State to prohibit this interfusion totally or partially: and if (as is the common view) we regard its rights over its territory as only limited by the duty of avoiding mischief to other States---according to the analogy of private rights of property---it must be competent for it to exclude inhabitants of other States altogether from its territory, without violation of duty. I conceive that this exclusive territorial dominion must be generally admitted---with a certain reservation in the case of States that claim the ownership of large tracts of unoccupied land---owing to the inconveniences and dangers of conflict that must generally attend any division or limitation of dominion: and, if so, a State must obviously have the right to admit aliens on its own terms, imposing any conditions on entrance or any tolls on transit, and subjecting them to any legal restrictions or disabilities that it may deem expedient. It ought not, indeed, having once admitted them, to apply to them suddenly, and without warning, a harsh differential treatment; but as it may legitimately exclude them altogether, it must have a right to treat them in any way it thinks fit, after due warning given and due time allowed for withdrawal. And if it may deal thus with aliens, it must clearly have similar rights in respect of its own alienated members. Doubtless such exclusive or differential treatment---unless justified as a necessary precaution against mischief to the State adopting it---is opposed to international morality as wantonly unfriendly, and would justify retaliatory exclusion, or other unfriendly acts, though not war. But it can hardly be regarded as even unfriendly for a government to apply to aliens within its territory the laws and administrative measures that it applies to its own subjects, even if they do not accord with the alien's view of justice. Thus a Frenchman, holding land in Ireland, could not reasonably complain of being judicially forced to give his tenants ``fair'' rents, though he might reasonably complain if such a measure were applied only to lands held by Frenchmen. If, however, the laws of one nation (A) are designedly made, or designedly administered, so as to cause special loss or annoyance to the members of another nation (B) residing in the territory of A, B has clearly a ground of complaint, and a claim to reparation if the unequal treatment is applied without warning. And even if such special loss or annoyance resulted without design, complaint would not be unreasonable; but if such complaint had no effect, it would not be in accordance with the principle of non-interference for B to take ulterior hostile measures beyond breaking off communication with A; and probably it would not be expedient to do more than warn its subjects against travelling or residing in A.