198 Reason and Its Rivals (ca. 1550– 1815)called the principle of necessity, meaning the right to take action in emer-
gencies that involved infringements of the rights of others.
More important yet, Vattel inaugurated a major shift in the law relating to
the conduct of war. Th is was to propose replacing the principle of military
necessity— which (as previously observed) was the lone rule on the conduct
of war in just- war doctrine— with a fi xed code of rules on warfare. Th e
code would be the product of the voluntary law rather than of natural law.
Instead of determining the lawfulness of belligerent action in terms of its
military value in the circumstances of each individual case, there would be
a set of objective rules to be applied mechanically in all situations. For this
innovation, Vattel deserves to be regarded as the leading fi gure in the mod-
ern law of war.
If Vattel may fairly be said to be the father of our modern approach to the
laws of war, it must be confessed that his off spring had a very long gestation
period. Until well into the nineteenth century, hardly any progress was ac-
tually made on draft ing a code of laws to govern the conduct of war, beyond
a general agreement on a bare handful of rules, such as a prohibition on the
use of poison or the employment of assassins. But Vattel nonetheless de-
serves recognition as the fi rst international-law writer to point out the direc-
tion in which the law would go in the future.G. F. von Martens
Th e most noteworthy writer in the tradition in the generation aft er Vattel
was the German author Georg Friedrich von Martens. Originally from
Hamburg, he became a professor of jurisprudence at the University of Göt-
tingen in 1783. We have earlier taken note of his work as the compiler of a
collection of Eu ro pe an treaties (in 1791). In the course of his career, he served
various governments (some of which he outlived). In 1808, he became a coun-
sellor of state to the elector of Hanover. Two years later, he served in the coun-
cil of state of the short- lived Kingdom of Westphalia (a Bonaparte creation).
He then returned to Hanoverian ser vice, as privy cabinet councillor in 1814
and later as a representative of Hanover in the diet of the post- 1815 German
Confederation.
Amid this activity, he found time to pen a general treatise on interna-
tional law that was strongly in the spirit of Vattel. Th e fi rst edition appeared