Complex sentence: The first mention of a Fatimid navy occurs in 912/3, when 15 vessels were sent against Tripoli, which had rebelled against Fatimid rule, only to be defeated by the ships of Tripoli's inhabitants.
Split into simpler sentences: The first mention of a Fatimid navy occurs in 912/3, when 15 vessels were sent against Tripoli, which had rebelled against Fatimid rule. This squadron was defeated by the ships of Tripoli's inhabitants.