660	RENAISSANCES
from the Han Empire's record in South China; for, though, in this
case likewise, the subjugation of the local sedentary barbarian high-
landers was by no means complete, it went sufficiently far to rule out
all possibility of dangerous combinations between unsubdued southern
barbarian highlanders and invading northern barbarian Nomads in the
crisis in which the strength of the Sinic World's southern citadel was
put to the test in the fourth century of the Christian Era, whereas, in
Roman North-West Africa, a local eruption of unsubdued barbarian
highlanders had been one of the regular incidents in each of the succes-
sive paroxysms of simultaneous concentric barbarian attacks on the
perimeter of the Roman Empire's hollow ring. The Bellum Gildonicum
had broken out in a.d. 398, thirty-one years before Genseric's passage
into North-West Africa from the European shore of the Straits of
Gibraltar; and in the previous paroxysm in the third century of the
Christian Era there had been a similar outbreak of dissident local
Berbers taking up arms on their own initiative.1
Another politico-geographical weakness of the Roman Empire was
that, if once an aggressive external enemy had succeeded in breaking
through its inevitably long-drawn-out and ill-sited artificial frontier
defences, the Roman armies found themselves hampered by the narrow-
ness of their manoeuvring room in their attempts to foil the invader by
a strategy of defence in depth2 which proved the salvation of the Han
Power's feeble 'Tsin* epigoni in a.d. 383.3 When once an assailant had
broken through the Roman limes, the odds would be heavily against
the Roman defence force in its subsequent efforts to prevent the invader
from reaching the coast; for the whole terrene perimeter of the Roman
Empire was so thin—even in those Gallic and Anatolian sectors which
were of twice the tenuous standard thickness4—that any breach of the
limes would bring the Mediterranean within the invader's range; and,
when once he had debouched on its shores, the conductivity of this
central pool of politically neutral navigable water would give any pawn
that launched a keel on it the range and versatility of a queen, and
would offer these facilities for a sudden vast increase in mobility to
Gothic, Vandal, or Arab piratical craft with the same undiscriminating
hospitality that it had shown to Roman warships and to Alexandrian
merchantmen. The maritime physiography of the Hellenic World thus
militated against a latter-day Roman Empire at bay5 as powerfully as
it had once told in favour of Roman empire-builders who had made
Rome's imperial fortune when their audacity in taking to an element
on which their Carthaginian adversaries had previously reigned supreme
had justified itself by its success in breaking through a sea-borne Punic
'wooden curtain*.6
These weaknesses in the structure of the Roman Empire proved, in
the event, to outweigh, in combination, one fortuitous advantage over
the Han Empire which the Roman Empire enjoyed thanks to an accident
 *	See V. y. 219.
 *	The Diocletianic Restoration's substitution of a system of defence in depth for a
broken cordon along the limes has been noticed in VI. vii. 322-3,
a See p. 655, above.	4 See VI. vii. 217.
s See VI. vii. 93.	6 See IX. viii. 428-9.