Putting Nature and Nations Asunder 165the principal requirement being a declaration of war. In such confi cts, both
belligerents are equally entitled, in principle, to exercise the rights of war
(such as killing and capturing enemy soldiers).
Concerning the conduct of war, the voluntary law of nations contained
various modifi cations of natural law. For example, the voluntary law (but
not the natural law) prohibited killing by use of poison in warfare. An-
other important modifi cation concerned the capture of enemy property.
Natural law placed two important limitations on this right: fi rst, that only
property belonging to actual wrongdoers could be taken; and second, that
the maximum total amount of property that could be taken must not exceed
the value of the damage done by the wrongdoer’s original injury. Th e volun-
tary law of nations relaxed both of these strictures. It allowed the capture of
property from any national on the enemy side, without regard to personal
wrongdoing. And it placed no ceiling on the amount of property that could
be taken.
Th e Impact of Grotius
If Grotius’s achievements appear, when viewed against the backdrop of
history as a whole, fairly modest, many later writers judged otherwise.
Th e German lawyer G. F. de Martens, writing in the late eigh teenth cen-
tury, hailed Grotius as the “father of... the law of nations, equally natu-
ral and positive.” Th e twentieth- century American legal phi los o pher
Roscoe Pound credited him— rather generously— with founding interna-
tional law “almost at one stroke.” Th ere may be exaggeration in these
assessments— perhaps leavened with ignorance of the prior work of Suárez—
but there is at least some truth in them, too. Grotius’s treatise became the
leading text from which practically all treatments of international law took
their departure.
More than any specifi c doctrines, Grotius bequeathed to his successors a
general outlook on the broad structure of international law. It was the dual-
ist perspective, with its distinction between natural law and the voluntary
law— that is, the laws of nature and of nations, respectively— that became
the basis of what has been called the “Grotian tradition” in international
law. Grotius had the remarkable posthumous fortune of being regarded as
a major progenitor of both of these branches of international law, revered