In the preceding section it has been shown that a State has absolute legal authority over all persons within its territorial jurisdiction, and over its own citizens wherever they may be. In the exercise, however, of this authority over persons within its territorial limits who are claimed as citizens by other States, that is, over resident aliens, or naturalized citizens whose native States do not recognize the right of expatriation, this legal power, though not subject to legal limitation, is actually subject to certain limitations which international custom has created. Thus each State demands that its subjects, when abroad, shall receive protection in life and property, and in their private rights be not unduly discriminated against by the foreign State in which they may happen to be. Also States do not permit the foreign States to require from their subjects the performance of duties that properly may be required only of citizens, as, for example, service in its army. Resident aliens may indeed be required to lend their assistance, by service in the militia and police forces, or in a posse comitatus, to put down domestic disorder; for, enjoying the protection of the local law, they may fairly be required to aid in overcoming resistance to its enforcement. But they may not be compelled to serve in the national military forces in cases of public war.

During the Civil War, Great Britain did not object to the enrollment in the local militia of her citizens domiciled in the United States; and in the case of one Scott, who had declared his intention of becoming an American citizen, refused to take any steps to prevent his enrollment in the army in the field. Great Britain, however, emphatically protested to the government of the Southern Confederacy against the conscription of her subjects in the Southern States. Several of the leading European powers protested against the attempt on the part of the United States to conscript into its armies domiciled aliens who had declared their intention of becoming American citizens, whereupon the United States granted to such aliens sixty-five days in which to leave the country, upon failure to do which they were held liable to conscription; and this arrangement was acquiesced in by the Powers concerned, though not without complaint that the principles of international comity were being violated. When, in 1873, the "State of Nicaragua attempted by an amendment to her Constitution to make foreigners liable to military and other public services, protests from the American Minister were made, in consequence of which the project was abandoned.