Complex sentence: The wars with the Ottomans lasted well into the 17th century and Senj spawned many an uskok, small units of men rowing swift boats, very efficient guerrilla forces.
Split into simpler sentences: The wars with the Ottomans lasted well into the 17th century. For the wars with the Ottomans they were important because they were small units of men rowing swift boats, and so they proved to be very efficient guerrilla forces.