Complex sentence: The supremacy of the Republic of Venice in the Adriatic and the attention to the threat posed by an expanding Ottoman Empire gave the Habsburgs little opportunity to enlarge the two cities and they remained separately administered and retained their own autonomy until into the 18th century.
Split into simpler sentences: The supremacy of the Republic of Venice in the Adriatic and the attention to the threat posed by an expanding Ottoman Empire gave the Habsburgs little opportunity to enlarge the two cities. They remained separately administered and retained their autonomy until into the 18th century.