Putting Nature and Nations Asunder 147dent sovereigns. As it happened, though, he did not develop this line of
thought with any clarity. Th at would be left to Suárez and Grotius, who
would thereby attain the parenthood of modern international law— an
honor that might have, but did not, fall to Bodin.Changes in the Law of War
Some important new ideas were also being advanced on the law of war. Th e
most momentous of these was a signifi cant modifi cation of one of the key
features of medieval just-war doctrine: the radical asymmetry between the
rights of the just and the unjust sides. Th ese fl owed logically from the under-
lying fact that a war could not be just on both sides. Th e principle of iusta
causa was strictly objective, so that whichever side was in the wrong had no
entitlement what ever to wage war. Th is principle underwent an important
change in the sixteenth century. Th e chief fi gure in this development was
Vitoria, who expounded upon the subject in one of his famous relections,
entitled De iure belli (On the Law of War), given in 1539— the same year as
his discourse on the American Indians. Th is relection provides the single
best summation of medieval just-war doctrine.
In this work, Vitoria broke new ground in his exposition of a doctrine of
“invincible ignorance.” In principle, he adhered to the traditional position
that, strictly speaking, a war could be just only on one side. But at the same
time, he conceded that, when there was genuine uncertainty as to which
side possessed the requisite iusta causa, and no prospect of an authoritative
ruling on the question— that is, a state of “invincible ignorance”— then the
war would be treated, for practical purposes, as if it were just on both sides.
Strictly speaking, the war would still not actually be just on both sides. But
the unjust side would be “excused from sin” if it was waging its struggle in
good faith. Th e important practical result would be that each side would be
regarded as having equal rights to exercise the normal prerogatives of just
belligerents.
Although Vitoria wrote on the laws of war, he could hardly claim to bring
practical experience to bear on the subject. He was, aft er all, a cloistered
monk and theology scholar. In the generations following him, three major
writers emerged who were considerably more experienced (if not necessarily
wiser) in the ways of the world. Th ese newer writers— all from secular