Complex sentence: The Yugoslav Navy began a blockade of the Croatian Adriatic coast on 17 September, further isolating Zadar, and the city's electricity supply was cut on the first day of the JNA attack.
Split into simpler sentences: The Yugoslav Navy started a blockade of the entire Croatian Adriatic coast on 17 September, further isolating Zadar. The electrical power supply to the city was also cut off on the first day of the JNA attack.