Complex sentence: Meanwhile the Greek navy rapidly occupied the Aegean islands still under Ottoman rule, while the Greek fleet after two victories established naval supremacy over the Aegean preventing the Turks from bringing reinforcements to the Balkans.
Split into simpler sentences: Meanwhile the Greek navy rapidly occupied the Aegean islands still under Ottoman rule. After two victories, the Greek fleet established naval supremacy over the Aegean preventing the Turks from bringing reinforcements to the Balkans.