148 Reason and Its Rivals (ca. 1550– 1815)backgrounds— were men of action and extensive practical experience, and
that showed through in their writings.
Th e fi rst of the trio was Pierino Belli, who hailed from Piedmont in north-
western Italy. He probably studied law at the University of Perugia. But he
gained signifi cant practical experience in matters of war aft er becoming
military auditor (i.e., a military judge and legal adviser) to Emperor Charles
V in 1535. He was then promoted to counselor of war by Charles’s son, King
Philip II of Spain. His principal work, On Military Matters and on War, was
written in 1558 and published fi ve years later (dutifully dedicated to Philip
II). In later years, Belli returned from Spain to his home region to serve the
Duke of Savoy in various diplomatic roles. His work had comparatively little
impact at the time. It was rediscovered in the nineteenth century (by the
prominent Italian legal scholar and statesman Pasquale Mancini) and
thereby became a retrospective classic of international law.
A more important fi gure was Balthasar Ayala. A native of Antwerp, then
part of the Habsburg domains, he was from a prominent family of Spanish
origin. Aft er legal studies at the University of Louvain, he became, in 1580,
an auditor in the Spanish army in the Netherlands. Th e following year, he
published the fi rst edition of On the Law of War and on the Duties Con-
nected with War and on Military Discipline. He adhered to the substitu-
tion theory of the ius gentium— holding the law of nature to be the law
governing humanity during its golden age (in the Garden of Eden) and the
ius gentium to be the law governing humanity in its fallen state. Conse-
quently, the institutions of war and slavery were clearly placed in the ius
gentium category.
Th e impact of the new tolerance for wars that were regarded as just on
both sides is evident in Ayala’s writing. He matter- of- factly reproduced Vi-
toria’s conclusions, though in slightly diff erent terms. He distinguished be-
tween two distinct legal concerns. One was the question of the justice of the
war itself, and the other was what he called the eff ects of war. For a war to be
just, it was necessary that the key criterion of iusta causa be objectively sat-
isfi ed, in the manner of traditional just-war doctrine. Regarding the eff ects
of war, however— meaning chiefl y the rights of war such as the right to cap-
ture and enslave enemy soldiers— there is equality between the two sides.
Th is marks the fi rst clear exposition of a principle of the law of war that re-
mains fundamental to the present day: that the rights and duties of the bel-