Complex sentence: At a very early date neighbours of the Corsairs of Almissa, the highlanders of the Poljica Principality (Poljička Republika), became their friends and allies, who were thus enabled to harass the seaborne trade without fear of a sudden attack by land.
Split into simpler sentences: At a very early date neighbours of the Corsairs of Almissa, the highlanders of the Poljica Principality (Poljička Republika), became their friends and allies. This allowed them to harass the seaborne trade, without fear of a sudden attack by land.