Complex sentence: Despite the subsequent loss of much of Italy to the Lombards, the Byzantines maintained control of the seas--as the Lombards rarely ventured to sea--and were thus able to retain several coastal strips of Italian territory for centuries.
Split into simpler sentences: Despite the subsequent loss of much of Italy to the Lombards, the Byzantines maintained control of the seas. As the Lombards rarely ventured to sea the Byzantines were thus able to retain several coastal strips of Italian territory for centuries.