Keeping Kings in Check 53than either church or empire. In its role as the foundation of international
law, its consequences are with us still.
We will look briefl y fi rst at the empire and the papacy, and then in some-
what more detail at the more salient features of natural law, and the impor-
tant changes that it underwent during the Middle Ages. Th ese changes en-
tailed some signifi cant rethinking of the relationship between natural law
and the ius gentium. Th ere will also be an exploration of the foremost
achievement— at least in intellectual terms— of medieval international law,
the development of just-war doctrine. Th ere will also be an all- too- brief look
at the practical, and extensive, body of law known as the ius commune,
which also became a fertile source of rules for the later law of nations.
Universal Empire
Th e collective medieval psyche was obsessed by the glorious memory of the
Roman Empire, and it is not hard to see why. Th e empire appeared to have
been incomparably greater and grander than the scattering of rude statelets
that succeeded it. Th e city of Rome contained— or even constituted— the
most striking evidence of this. With its population reduced by some 90 per-
cent from imperial times, those remaining seemed to be living, huddled in
squalor, amid a forest of spectacular buildings and monuments, all falling
steadily into ruin. It was small wonder that people pined for the great days
of the past.
When Charlemagne was crowned emperor in Rome in 800, some regarded
this as a reconstituting of the Roman imperial line. Charlemagne and his
Frankish successors, however, made little of their ties to ancient Rome. It was
only when the empire was reconstituted yet again, by the German ruler Otto
I in 961, that the real continuity with the Roman Empire began to assume
legal and po liti cal signifi cance, with regular installations of the emperors in
Rome by the popes of the day. Th at the Roman emperor— who became offi -
cially “Holy” in 1157— was the foremost ruler in Western Eu rope was not
doubted. But the claims that were advanced on his behalf were truly star-
tling: of universal dominion.
It was canon lawyers who fi rst asserted that the Holy Roman emperor was
the lord of the entire world (dominu s mundi) and that, as such, he possessed
a residual de jure sovereignty over the entire world. Rulers of the various