164 Reason and Its Rivals (ca. 1550– 1815)framework of international law, but it was Grotius who applied that frame-
work in detail in the concrete context of warfare. His second major contri-
bution was to identify what became the canonical just causes of war from
the standpoint of natural law.
Th e just causes of war according to natural law were identifi ed by Grotius
as being three in number. First was defense. It must be emphasized that
this did not mean self- defense, in the sense of repelling an actual attack. It
meant preemptive action to counteract an impending attack or serious threat
of some kind. Th e second just cause was the obtaining of something that
was owing by another state but was being withheld. An example would be a
case in which part of a state’s territory was being occupied by a foreign power.
Force could be used to recover it. Th is second category also encompassed a
use of force to obtain compensation owing for a past wrong. Th e third just
cause was punishment of another state for past misconduct.
Th e heritage of medieval just-war doctrine is clear in all of this. Especially
clear is that Grotius’s idea of just war, like its medieval pre de ces sor, con-
cerned the justice of resorting to off ensive war, in the sense of entitling the
just side to strike the fi rst blow. Even the category of defensive war is off en-
sive in this sense, since it envisages that the just side would mount a preemp-
tive attack— that is, would strike the fi rst blow— to ward off the threat that
was impending. As a consequence, self- defense in the face of an actual, on-
going attack was not within any of the types of just war.
Also outside the category of just war was another type of action that later
lawyers would refer to as humanitarian intervention: the use of armed force
to prevent a tyrannical ruler from oppressing his own subjects. Th is falls
outside the category of punitive war because it is envisaged that the armed
force will be directed not toward the chastisement of the wicked prince
(much though he may deserve it), but rather toward relieving the suff ering of
the oppressed subjects. Grotius favored allowing the use of force for this no-
ble purpose, while holding back from qualifying it as a war. Nevertheless,
it may be regarded as, in substance, a sort of quasi- just war.
Ve r y d i ff erent was the treatment of just war from the standpoint of the
voluntary law of nations. In the eyes of the voluntary law of nations, there
was no bar, even in principle, to a war’s being just on both sides. So there
was no need to make use of Vitoria’s principle of ignorance. Th e criteria for
a just war under the voluntary law were formal rather than substantive, with