48 AHISTORY OFTHE CRUSADES I 
 
 
to give protection to the coastal bases. From them the Saracens repeatedly
raided the interior and threatened Monte CassinO and San Vincenzo~ King Louis,
called in by the monasteries in 852, again failed before the walled cities.
Within the same decade the threatened monasteries bought off other Saracen
bands, and cities like Naples and Capua were plundered, all the duchy of
Benevento was overrun, and most of Campania also. As long as the Saracens
held their naval bases, they remained a threat, since neither the imperial
nor the ducal forces were willing or able to drive them out. 
 Finally, in 866 Louis II, now emperor, heeded the persistent pleas of Benevento
and Capua. He recruited large forces in north and central Italy and compelled
the south Italian dukes and cities to abandon their local rivalries and to
join him in a full-scale campaign against the Saracens at Ban. He carried
out a methodical, but often interrupted, plan of attrition against the enemy
by destroying or occupying the fortress towns in the approaches to the naval
base. Canosa, Venosa, and Matera were occupied, but again he could not take
Ban because of the lack of sea power. In 868 a large Byzantine fleet did
appear before the city, but then the imperial land forces were inadequate
and the four hundred Byzantine ships sailed back to Corinth when negotiations
for the marriage between Louis's daughter and Basil I's son failed to reach
a satisfactory conclusion. A Venetian force, however, crippled a Saracen
fleet off the port of Taranto in 867. The emperor also had to protect his
land forces against attack from the rear by those Saracens who were coming
into Italy through Naples, since there duke Sergius II, in order to maintain
his independence of the emperor, had aligned himself with the enemy. However,
the emperor was fortunate in having the active support of Venice and the
Dalmatian towns. While their naval forces blockaded the port, he attacked
the city on the land side. After four years of intermittent warfare the emperor
successfully concluded the campaign by taking Ban in 871. It was a decisive
blow to the Arabs and initiated the gradual lessening of their power on the
mainland. But the leadership and success of the emperor Louis were repaid
with treachery. Sergius of Naples, Waifar of Salerno, Lambert of Spoleto,
and Adelchis of Benevento conspired against him, their henchmen ambushed
him, and they held him prisoner till he swore never to return to southern
Italy. In that way they hoped to maintain their independence of imperial
sovereignty. But when a force of 30,000 African Saracens threatened Salerno
it was another story. In 872 the traitors again