Complex sentence: The first battles of the campaign ended in defeat for the Byzantines; the Persian army fought their way to the Bosphorus; however, because Constantinople was protected by impenetrable walls and a strong navy, Heraclius was able to avoid total defeat.
Split into simpler sentences: The first battles of the campaign ended in defeat for the Byzantines; the Persian army fought their way to the Bosphorus. However, because Constantinople was protected by impenetrable walls and a strong navy, Heraclius was able to avoid total defeat.