Complex sentence: Several tens of thousands of Balkan Muslims were streaming into the Empire as refugees, while tensions mounted with the Kingdom of Greece over possession of the islands of the northeastern Aegean, which Greece had captured during the wars.
Split into simpler sentences: Several tens of thousands of Balkan Muslims were streaming into the Empire as refugees. At the same time tensions mounted with the Kingdom of Greece over possession of the islands of the northeastern Aegean, which Greece had captured during the wars.