Complex sentence: That year, Croatia militarily defeated Croatian Serbs in offensives codenamed Flash and Storm in May and August respectively, leaving the area around Vukovar, the eastern Slavonia, as the single Croatian Serb-held area.
Split into simpler sentences: That year, Croatia militarily defeated Croatian Serbs in offensives codenamed Flash and Storm in May and August respectively. That left the area around Vukovar, the eastern Slavonia, as the single Croatian Serb-held area.