it away. War gives no other right over
prisoners than to disable them from doing any further harm,
by securing their persons. All nations concur in detesting
the murdering of prisoners in cold blood."[79]

Thus, it is now the established Law of Nations, that necessity is the
measure of violence in war, and humanity, its tempering spirit; or, as
it has been otherwise enunciated, the rights of war are to be measured
by the objects of the war.

Although we have a right to kill our enemies in war; it is only when
we find gentler methods insufficient to conquer their resistance and
bring them to terms, that we have a right to put them to death.[80]

Under the name of enemies are comprehended not only the first author
of the war, but also those who join him and support his cause.

[Sidenote: Cartel]

Out of these enlightened views of war has sprung the System of Cartels
for the exchange of prisoners. These exchanges are generally regulated
by special convention between the hostile states. Prisoners are
sometimes permitted to return home, upon condition not to serve again
during the war, or until duly exchanged. Officers are frequently
released upon their parole, on the same condition; and to carry more
effectually into operation the arrangements necessary for these
purposes, commissaries are permitted to reside in the respective
hostile states.

Subject to the principle of non-resistance, there are several classes
of persons that are generally considered exempt from the operations of
war, beyond the effects of unavoidable accident. "All the members of
the enemy's state," says Wheaton,

"may lawfully be treated as enemies, in a Public War; but it
does not follow that all are to be treated alike; though we
may lawfully destroy some of them, it does not follow that
we may lawfully destroy all; for the general rule derived
from the natural law is still the same, that no force
against an enemy is lawful, unless it is necessary to
accomplish the purposes of war. The custom of civilised
nations founded on this principle, has therefore exempted
the persons of the Sovran and his family, the members of the
Civil Government, women and children, cultivators of the
earth, artizans, labourers, merchants, men of science and
letters, and generally all other public or private persons
engaged in the ordinary civil pursuits of life, from the
direct ef